Accommodate SQLAlchemy 1.4/2.0

To accommodate SQLAlchemy 1.4 and 2.0, the migration model now no longer
assumes that the SQLAlchemy Connection will autocommit an individual
operation.   This essentially means that for databases that use
non-transactional DDL (pysqlite current driver behavior, MySQL), there is
still a BEGIN/COMMIT block that will surround each individual migration.
Databases that support transactional DDL should continue to have the
same flow, either per migration or per-entire run, depending on the
value of the :paramref:`.Environment.configure.transaction_per_migration`
flag.

Compatibility is established such that the entire library should
not generate any SQLAlchemy 2.0 deprecation warnings and
SQLALCHEMY_WARN_20 is part of conftest.py. (one warning remains
for the moment that needs to be resolved on the SQLAlchemy side)

The test suite requires SQLAlchemy 1.4.0b2 for testing 1.4;
1.4.0b1 won't work.

Test suite / setup also being modernized, as we are at
SQLAlchemy 1.3 we can now remove the majority of the testing
suite plugin.

Change-Id: If55b1ea3c12ead66405ab3fadc76d15d89dabb90

3 files changed, 24 insertions, 18 deletions

diff --git a/alembic/script/base.py b/alembic/script/base.py
index fea9e87..363895c 100644
--- a/alembic/script/base.py
+++ b/alembic/script/base.py
@@ -171,7 +171,7 @@ class ScriptDirectory(object):
                     "ancestor/descendant revisions along the same branch"
                 )
             ancestor = ancestor % {"start": start, "end": end}
-            compat.raise_from_cause(util.CommandError(ancestor))
+            compat.raise_(util.CommandError(ancestor), from_=rna)
         except revision.MultipleHeads as mh:
             if not multiple_heads:
                 multiple_heads = (
@@ -185,15 +185,15 @@ class ScriptDirectory(object):
                 "head_arg": end or mh.argument,
                 "heads": util.format_as_comma(mh.heads),
             }
-            compat.raise_from_cause(util.CommandError(multiple_heads))
+            compat.raise_(util.CommandError(multiple_heads), from_=mh)
         except revision.ResolutionError as re:
             if resolution is None:
                 resolution = "Can't locate revision identified by '%s'" % (
                     re.argument
                 )
-            compat.raise_from_cause(util.CommandError(resolution))
+            compat.raise_(util.CommandError(resolution), from_=re)
         except revision.RevisionError as err:
-            compat.raise_from_cause(util.CommandError(err.args[0]))
+            compat.raise_(util.CommandError(err.args[0]), from_=err)
 
     def walk_revisions(self, base="base", head="heads"):
         """Iterate through all revisions.
@@ -571,7 +571,7 @@ class ScriptDirectory(object):
         try:
             Script.verify_rev_id(revid)
         except revision.RevisionError as err:
-            compat.raise_from_cause(util.CommandError(err.args[0]))
+            compat.raise_(util.CommandError(err.args[0]), from_=err)
 
         with self._catch_revision_errors(
             multiple_heads=(
@@ -659,7 +659,7 @@ class ScriptDirectory(object):
         try:
             script = Script._from_path(self, path)
         except revision.RevisionError as err:
-            compat.raise_from_cause(util.CommandError(err.args[0]))
+            compat.raise_(util.CommandError(err.args[0]), from_=err)
         if branch_labels and not script.branch_labels:
             raise util.CommandError(
                 "Version %s specified branch_labels %s, however the "
diff --git a/alembic/script/revision.py b/alembic/script/revision.py
index 683d322..c75d1c0 100644
--- a/alembic/script/revision.py
+++ b/alembic/script/revision.py
@@ -422,9 +422,12 @@ class RevisionMap(object):
         except KeyError:
             try:
                 nonbranch_rev = self._revision_for_ident(branch_label)
-            except ResolutionError:
-                raise ResolutionError(
-                    "No such branch: '%s'" % branch_label, branch_label
+            except ResolutionError as re:
+                util.raise_(
+                    ResolutionError(
+                        "No such branch: '%s'" % branch_label, branch_label
+                    ),
+                    from_=re,
                 )
             else:
                 return nonbranch_rev
diff --git a/alembic/script/write_hooks.py b/alembic/script/write_hooks.py
index 7d0843b..d6d1d38 100644
--- a/alembic/script/write_hooks.py
+++ b/alembic/script/write_hooks.py
@@ -39,9 +39,10 @@ def _invoke(name, revision, options):
     """
     try:
         hook = _registry[name]
-    except KeyError:
-        compat.raise_from_cause(
-            util.CommandError("No formatter with name '%s' registered" % name)
+    except KeyError as ke:
+        compat.raise_(
+            util.CommandError("No formatter with name '%s' registered" % name),
+            from_=ke,
         )
     else:
         return hook(revision, options)
@@ -65,12 +66,13 @@ def _run_hooks(path, hook_config):
         opts["_hook_name"] = name
         try:
             type_ = opts["type"]
-        except KeyError:
-            compat.raise_from_cause(
+        except KeyError as ke:
+            compat.raise_(
                 util.CommandError(
                     "Key %s.type is required for post write hook %r"
                     % (name, name)
-                )
+                ),
+                from_=ke,
             )
         else:
             util.status(
@@ -89,12 +91,13 @@ def console_scripts(path, options):
 
     try:
         entrypoint_name = options["entrypoint"]
-    except KeyError:
-        compat.raise_from_cause(
+    except KeyError as ke:
+        compat.raise_(
             util.CommandError(
                 "Key %s.entrypoint is required for post write hook %r"
                 % (options["_hook_name"], options["_hook_name"])
-            )
+            ),
+            from_=ke,
         )
     iter_ = pkg_resources.iter_entry_points("console_scripts", entrypoint_name)
     impl = next(iter_)