Add tests for ClassVar-annotated Migration fields (typeddjango#2702)

For the fields "initial" and "atomic", this is strictly correct, as these are
set on the class, not the instance.

For "operations", "dependencies", "run_before", and "replaces", Django
intentionally copies the class attributes to the instance during construction.
However, annotating the attributes as either (using "|") class or instance
variables causes MyPy to issue the warning: "Cannot override instance variable
(previously declared on base class "Migration") with class variable". Since
Django projects will nearly always override these as class variables, annotate
them as such.

Signed-off-by: Brian Helba <[email protected]>
Signed-off-by: Yury V. Zaytsev <[email protected]>
Co-authored-by: Brian Helba <[email protected]>

Author: 2 people

tests/typecheck/db/migrations/test_classvar.yml

@@ -0,0 +1,28 @@
+- case: explicit
+  main: |
+    from typing import ClassVar, List
+    from django.db import migrations
+    from django.db.migrations.operations.base import Operation
+
+    class Migration(migrations.Migration):
+        operations: ClassVar[List[Operation]] = []
+        initial: ClassVar[bool] = True
+
+- case: explicit_incorrect
+  expect_fail: true
+  main: |
+    from typing import ClassVar, List
+    from django.db import migrations
+    from django.db.migrations.operations.base import Operation
+
+    class Migration(migrations.Migration):
+        operations: List[Operation] = []
+        initial: bool = True
+
+- case: implicit
+  main: |
+    from django.db import migrations
+
+    class Migration(migrations.Migration):
+        operations = []
+        initial = True