Create initial future package, RemovedIn20Warning

Reorganization of Select() is the first major element
of the 2.0 restructuring.   In order to start this we need
to first create the new Select constructor and apply legacy
elements to the old one.    This in turn necessitates
starting up the RemovedIn20Warning concept which itself
need to refer to "sqlalchemy.future", so begin to establish
this basic framework.   Additionally, update the
DML constructors with the newer no-keyword style.  Remove
the use of the "pending deprecation" and fix Query.add_column()
deprecation which was not acting as deprecated.

Fixes: #4845
Fixes: #4648
Change-Id: I0c7a22b2841a985e1c379a0bb6c94089aae6264c

1 files changed, 15 insertions, 2 deletions

diff --git a/lib/sqlalchemy/exc.py b/lib/sqlalchemy/exc.py
index c8e71dda5..cc096ad03 100644
--- a/lib/sqlalchemy/exc.py
+++ b/lib/sqlalchemy/exc.py
@@ -587,11 +587,24 @@ class NotSupportedError(DatabaseError):
 
 
 class SADeprecationWarning(DeprecationWarning):
-    """Issued once per usage of a deprecated API."""
+    """Issued for usage of deprecated APIs."""
+
+
+class RemovedIn20Warning(SADeprecationWarning):
+    """Issued for usage of APIs specifically deprecated in SQLAlchemy 2.0.
+
+    .. seealso::
+
+        :ref:`error_b8d9`.
+
+    """
 
 
 class SAPendingDeprecationWarning(PendingDeprecationWarning):
-    """Issued once per usage of a deprecated API."""
+    """A similar warning as :class:`.SADeprecationWarning`, this warning
+    is not used in modern versions of SQLAlchemy.
+
+    """
 
 
 class SAWarning(RuntimeWarning):