Make BufferedReader generic over a protocol

[srittau]

Works towards #13495

[github-actions]

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

streamlit (https://github.com/streamlit/streamlit)
- lib/tests/streamlit/elements/audio_test.py:78:38: error: Argument 1 to "_calculate_file_id" has incompatible type "Union[str, Path, bytes, BytesIO, RawIOBase, BufferedReader, Any, None]"; expected "bytes"  [arg-type]
+ lib/tests/streamlit/elements/audio_test.py:78:38: error: Argument 1 to "_calculate_file_id" has incompatible type "Union[str, Path, bytes, BytesIO, RawIOBase, BufferedReader[_BufferedReaderStream], Any, None]"; expected "bytes"  [arg-type]

werkzeug (https://github.com/pallets/werkzeug)
- tests/test_wsgi.py:158: error: Incompatible types in assignment (expression has type "TextIOWrapper[BufferedReader]", variable has type "LimitedStream")  [assignment]
+ tests/test_wsgi.py:158: error: Incompatible types in assignment (expression has type "TextIOWrapper[BufferedReader[LimitedStream]]", variable has type "LimitedStream")  [assignment]
+ tests/test_wsgi.py:158: error: Value of type variable "_BufferedReaderStreamT" of "BufferedReader" cannot be "LimitedStream"  [type-var]

urllib3 (https://github.com/urllib3/urllib3)
+ test/test_response.py:770: error: Unused "type: ignore" comment  [unused-ignore]
+ test/test_response.py:770: error: Value of type variable "_BufferedReaderStreamT" of "BufferedReader" cannot be "HTTPResponse"  [type-var]
+ test/test_response.py:770: note: Error code "type-var" not covered by "type: ignore" comment
+ test/test_response.py:782: error: Unused "type: ignore" comment  [unused-ignore]
+ test/test_response.py:782: error: Value of type variable "_BufferedReaderStreamT" of "BufferedReader" cannot be "HTTPResponse"  [type-var]
+ test/test_response.py:782: note: Error code "type-var" not covered by "type: ignore" comment
+ test/test_response.py:787: error: Unused "type: ignore" comment  [unused-ignore]
+ test/test_response.py:787: error: Value of type variable "_BufferedReaderStreamT" of "BufferedReader" cannot be "HTTPResponse"  [type-var]
+ test/test_response.py:787: note: Error code "type-var" not covered by "type: ignore" comment
+ test/test_response.py:800: error: Unused "type: ignore" comment  [unused-ignore]
+ test/test_response.py:800: error: Value of type variable "_BufferedReaderStreamT" of "BufferedReader" cannot be "HTTPResponse"  [type-var]
+ test/test_response.py:800: note: Error code "type-var" not covered by "type: ignore" comment

materialize (https://github.com/MaterializeInc/materialize)
- misc/python/materialize/mzbuild.py:828: error: Incompatible types in assignment (expression has type "BufferedRandom", variable has type "BufferedReader")  [assignment]
+ misc/python/materialize/mzbuild.py:828: error: Incompatible types in assignment (expression has type "BufferedRandom", variable has type "BufferedReader[_BufferedReaderStream]")  [assignment]

[srittau]

Looking at the primer hits: Both LimitedStream (from werkzeug) and HTTPResponse (from urllib3) have wrong annotations for their readinto() methods: BufferedReader (both, the C and Python implementation) sends a memoryview to self.raw.readinto(), while the libraries's readinto() methods are annotated as accepting only bytearray. But their implementation actually accept memoryviews:

• https://github.com/pallets/werkzeug/blob/7868bef5d978093a8baa0784464ebe5d775ae92a/src/werkzeug/wsgi.py#L520-L566
• https://github.com/urllib3/urllib3/blob/04662c9ae08c9f63fa254772d7618db65123a35e/src/urllib3/response.py#L509-L515