Render N'' for SQL Server unicode literals

The ``literal_processor`` for the :class:`.Unicode` and
:class:`.UnicodeText` datatypes now render an ``N`` character in front of
the literal string expression as required by SQL Server for Unicode string
values rendered in SQL expressions.

Note that this adds full unicode characters to the standard test suite,
which means we also need to bump MySQL provisioning up to utf8mb4.
Modern installs do not seem to be reproducing the 1271 issue locally,
if it reproduces in CI it would be better for us to skip those ORM-centric
tests for MySQL.

Also remove unused _StringType from SQL Server dialect

Fixes: #4442
Change-Id: Id55817b3e8a2d81ddc8b7b27f85e3f1dcc1cea7e

1 files changed, 3 insertions, 6 deletions

diff --git a/lib/sqlalchemy/testing/provision.py b/lib/sqlalchemy/testing/provision.py
index 88dc28528..70ace0511 100644
--- a/lib/sqlalchemy/testing/provision.py
+++ b/lib/sqlalchemy/testing/provision.py
@@ -207,15 +207,12 @@ def _mysql_create_db(cfg, eng, ident):
         except Exception:
             pass
 
-        # using utf8mb4 we are getting collation errors on UNIONS:
-        # test/orm/inheritance/test_polymorphic_rel.py"
-        # 1271, u"Illegal mix of collations for operation 'UNION'"
-        conn.execute("CREATE DATABASE %s CHARACTER SET utf8mb3" % ident)
+        conn.execute("CREATE DATABASE %s CHARACTER SET utf8mb4" % ident)
         conn.execute(
-            "CREATE DATABASE %s_test_schema CHARACTER SET utf8mb3" % ident
+            "CREATE DATABASE %s_test_schema CHARACTER SET utf8mb4" % ident
         )
         conn.execute(
-            "CREATE DATABASE %s_test_schema_2 CHARACTER SET utf8mb3" % ident
+            "CREATE DATABASE %s_test_schema_2 CHARACTER SET utf8mb4" % ident
         )