Reject generator functions in sync_to_async

[lin-hongkuan]

Summary

• reject synchronous and asynchronous generator functions in sync_to_async() with the existing TypeError path
• apply the same detection to callable instances via their __call__ method
• add regression tests for generator functions and generator callable instances

sync_to_async() currently calls a generator function in the worker thread only far enough to create the generator object. Iterating that generator then runs the body in the caller thread, which is the confusing behavior reported in the issue. Raising early is safer than returning an object that does not actually execute under the adapter.

Fixes #38

Tests

• python -m pytest tests\test_sync.py -q
• python -m pytest -q
• python -m mypy asgiref tests\test_sync.py
• git diff --check