add PlaceholderNode when deferring manager class hook

[asottile]

see #2224

[asottile]

ok so this passes the current tests -- but how do I demonstrate the improvement is tricky. I need to somehow make a circular Manager <-> QuerySet ? would appreciate some help on writing a test for this :S

[asottile]

test originally failing with:

_______________________ test_from_queryset_with_deferral _______________________
/home/asottile/workspace/django-stubs/tests/typecheck/managers/querysets/test_from_queryset.yml:853: 
E   pytest_mypy_plugins.utils.TypecheckAssertionError: Invalid output: 
E   Actual:
E     main:2: note: Revealed type is "Any"          (diff)
E    main:3: note: Revealed type is "builtins.str"
E     myapp/models:15: error: Variable "myapp.models._base" is not valid as a type  [valid-type] (diff)
E     myapp/models:15: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases (diff)
E     myapp/models:15: error: Invalid base class "_base"  [misc] (diff)
E   Expected:
E     main:2: note: Revealed type is "builtins.int" (diff)
E    main:3: note: Revealed type is "builtins.str"
E   Alignment of first line difference:
E     E: ...te: Revealed type is "builtins.int"
E     A: ...te: Revealed type is "Any"
E                                 ^

[asottile]

this is needed to skip the importing of models -- since the forced-deferral trick models.py triggers a NameError when actually imported

[flaeppe]

Nice work. Great that you figured out a test case