[channels] Compatibility fixes for django-stubs 6.0.3

[srittau]

LazyObject is generic since django-stubs 6.0.3.

This fixes problems in unrelated PRs and preempts upcoming daily CI errors.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mypy (https://github.com/python/mypy)
  error: INTERNAL ERROR -- Please try using mypy master on GitHub:

[AlexWaygood]

Diff from mypy_primer, showing the effect of this PR on open source code:

mypy (https://github.com/python/mypy)
  error: INTERNAL ERROR -- Please try using mypy master on GitHub:

When did we introduce a mypy crash? That seems pretty concerning. Doesn't look like it's related to this PR, but we should probably try to fix that 😬

[srittau]

When did we introduce a mypy crash? That seems pretty concerning. Doesn't look like it's related to this PR, but we should probably try to fix that 😬

Unclear, but it's fairly recent.

[brianschubert]

Looks like the mypy crash is due to mypy now setting num_workers = 4 in mypy_self_check.ini (python/mypy#21221), which is incompatible with disabling the cache directory as is done in mypy_primer.

Traceback for v1.20.0:

$ uv run python3 -m mypy_primer --new 1.20.0 --old 1.20.0 -k 'mypy$'
...
new
> /tmp/mypy_primer/mypy_new/venv/bin/mypy --config-file mypy_self_check.ini -p mypy -p mypyc --python-executable=/tmp/mypy_primer/projects/_mypy_venv/bin/python --warn-unused-ignores --warn-redundant-casts --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit=-1 (10.6s)
UNEXPECTED FAILURE
        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: 1.20.0
        note: use --pdb to drop into pdb
        Traceback (most recent call last):
          File "/__main__.py", line 15, in console_entry
            main()
            ~~~~^^
          File "mypy/main.py", line 141, in main
            return
          File "mypy/main.py", line 225, in run_build
            show_messages(new_messages, f, formatter, options)
          File "mypy/build.py", line 384, in build
            # Create metastore before workers to avoid race conditions.
          File "mypy/build.py", line 298, in connect
            try:
        TypeError: mypy.ipc.BadStatus object expected; got AssertionError

Mypy master seems to fix the crash and shows a more explicit error:

$ uv run python3 -m mypy_primer --new master --old master -k 'mypy$'
...
new
> /tmp/mypy_primer/mypy_new/venv/bin/mypy --config-file mypy_self_check.ini -p mypy -p mypyc --python-executable=/tmp/mypy_primer/projects/_mypy_venv/bin/python --warn-unused-ignores --warn-redundant-casts --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit=-1 (0.5s)
UNEXPECTED FAILURE
        error: cache must be enabled in parallel mode