Fix mypy crash on QuerySet subclass with forward-referenced TypeVar

[UnknownPlatypus]

Prior to the plugin code change, a fresh mypy --strict tests/assert_type was crashing like so:

tests/assert_type/db/models/test_managers.py:15: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.19.1
Traceback (most recent call last):
  File "mypy/semanal.py", line 7568, in accept
  File "mypy/nodes.py", line 995, in accept
  File "mypy/semanal.py", line 978, in visit_func_def
  File "mypy/semanal.py", line 1023, in analyze_func_def
  File "mypy/semanal.py", line 7214, in defer
AssertionError: Must not defer during final iteration
tests/assert_type/db/models/test_managers.py:15: : note: use --pdb to drop into pdb

Fixes #2911