Remove typing.AwaitableGenerator

[srittau]

Part of #7580

[github-actions]

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

trio (https://github.com/python-trio/trio)
+ src/trio/_core/_traps.py:65: 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
+ src/trio/_core/_traps.py:65: : note: use --pdb to drop into pdb
- src/trio/_socket.py:244: error: Argument 1 to "getaddrinfo" of "HostnameResolver" has incompatible type "bytes | str | None"; expected "bytes | None"  [arg-type]
- src/trio/_path.py:267: error: Argument 1 to "as_uri" of "Path" has incompatible type "trio._path.Path"; expected "pathlib.Path"  [arg-type]
- src/trio/_tests/test_exports.py:163: error: Unused "type: ignore" comment  [unused-ignore]
- src/trio/_core/_tests/test_asyncgen.py:309: error: Item "None" of "FrameType | None" has no attribute "f_locals"  [union-attr]
- src/trio/_core/_tests/test_asyncgen.py:313: error: Item "None" of "FrameType | None" has no attribute "f_locals"  [union-attr]
+ Traceback (most recent call last):
+   File "mypy/checker.py", line 643, in accept
+   File "mypy/nodes.py", line 1155, in accept
+   File "mypy/checker.py", line 5462, in visit_decorator
+   File "mypy/checker.py", line 5469, in visit_decorator_inner
+   File "mypy/checker.py", line 1239, in check_func_item
+   File "mypy/checker.py", line 1403, in check_func_def
+   File "mypy/checker.py", line 7549, in named_generic_type
+   File "mypy/checker.py", line 7556, in lookup_typeinfo
+   File "mypy/checker.py", line 7639, in lookup_qualified
+ KeyError: "Could not find builtin symbol 'AwaitableGenerator' (If you are running a test case, use a fixture that defines this symbol)"

discord.py (https://github.com/Rapptz/discord.py)
- ...typeshed_to_test/stdlib/typing.pyi:1046: note: "update" of "TypedDict" defined here
+ ...typeshed_to_test/stdlib/typing.pyi:1034: note: "update" of "TypedDict" defined here