establish mypy / typing approach for v2.0

large patch to get ORM / typing efforts started.
this is to support adding new test cases to mypy,
support dropping sqlalchemy2-stubs entirely from the
test suite, validate major ORM typing reorganization
to eliminate the need for the mypy plugin.

* New declarative approach which uses annotation
  introspection, fixes: #7535
* Mapped[] is now at the base of all ORM constructs
  that find themselves in classes, to support direct
  typing without plugins
* Mypy plugin updated for new typing structures
* Mypy test suite broken out into "plugin" tests vs.
  "plain" tests, and enhanced to better support test
  structures where we assert that various objects are
  introspected by the type checker as we expect.
  as we go forward with typing, we will
  add new use cases to "plain" where we can assert that
  types are introspected as we expect.
* For typing support, users will be much more exposed to the
  class names of things.  Add these all to "sqlalchemy" import
  space.
* Column(ForeignKey()) no longer needs to be `@declared_attr`
  if the FK refers to a remote table
* composite() attributes mapped to a dataclass no longer
  need to implement a `__composite_values__()` method
* with_variant() accepts multiple dialect names

Change-Id: I22797c0be73a8fbbd2d6f5e0c0b7258b17fe145d
Fixes: #7535
Fixes: #7551
References: #6810

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/fixtures.py b/lib/sqlalchemy/testing/fixtures.py
index ecc20f163..7228e5afe 100644
--- a/lib/sqlalchemy/testing/fixtures.py
+++ b/lib/sqlalchemy/testing/fixtures.py
@@ -20,6 +20,7 @@ from .util import drop_all_tables_from_metadata
 from .. import event
 from .. import util
 from ..orm import declarative_base
+from ..orm import DeclarativeBase
 from ..orm import registry
 from ..schema import sort_tables_and_constraints
 
@@ -82,6 +83,21 @@ class TestBase:
         yield reg
         reg.dispose()
 
+    @config.fixture
+    def decl_base(self, metadata):
+        _md = metadata
+
+        class Base(DeclarativeBase):
+            metadata = _md
+            type_annotation_map = {
+                str: sa.String().with_variant(
+                    sa.String(50), "mysql", "mariadb"
+                )
+            }
+
+        yield Base
+        Base.registry.dispose()
+
     @config.fixture()
     def future_connection(self, future_engine, connection):
         # integrate the future_engine and connection fixtures so