gh-158952: Reject read-only buffers before reading in _pyio

[emerardd]

Passing a read-only destination to _pyio.BytesIO.readinto() or readinto1() currently consumes input before raising TypeError. The C implementation rejects the argument without moving the stream position. For example, readinto(b"xxx") on _pyio.BytesIO(b"abcdef") raises but leaves only b"def" unread.

Check the destination memoryview's readonly flag in BufferedIOBase._readinto() before calling read() or read1(). This prevents the pure Python fallback from consuming input on an invalid destination.

The shared C/Python BytesIO regression test covers both methods, bytes and read-only memoryviews, empty destinations, and initial positions at the beginning, near EOF, and beyond EOF. It checks both the position and the remaining input after the error.

Validation on a locally built Windows x64 debug CPython 3.16.0a0 at main 2639fd65ff8e0c1949c480a8e670fe9c2467a1f8:

• Before the fix, the C test passes and the Python test fails eight subcases.
• With the fix, both tests pass.
• python_d.exe -m test -v test_io: 1,043 tests run, 35 skipped, success.
• Ruff 0.15.17 and git diff --check pass.

Linux and macOS validation has not been run locally.

Fixes #158952.

• Issue: _pyio.BytesIO.readinto() advances the stream when passed a read-only buffer #158952

[picnixz]

Use support.subTests() for parametrizing the method and buffer. It would be easier.

[emerardd]

Updated the test to use support.subTests() in 3831e0c, keeping all parameter combinations. The full test_io suite passes.

[cmaloney]

Overall looking good to me, I think the test should go a more general location for buffered I/O classes

[cmaloney]

I think this would fit better in test_bufferedio. Goal for me is to test all the Buffered I/O implementations readinto a readonly buffer doesn't advance. That the others implement it right already is nice, but a good thing to make sure we keep.

test_general IOTest would also work well as a spot but I don't like it quite as much just because it keeps adding to the big ball of general tests.