b"".join should release GIL in case of memoryview

Bug report

Bug description:

CPython PR #17757 allowed the GIL to be released during b"".join() operations. However, this optimization currently only triggers if every passed object is of type bytes; it falls back to acquiring the GIL if it encounters mutable buffers. While reacquiring the GIL is necessary for mutable types like bytearray, we can still safely drop the GIL for memoryview objects that expose immutable bytes.

Please refer to https://github.com/fsspec/gcsfs/pull/795#discussion_r3032749881 for more context.

cc: @martindurant

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-148456

先查看 CPython PR #17757 和相關的 PR gh-148456，接著追蹤針對 memoryview 物件的 b"".join 處理方式。完成的標準是：不可變位元組 memoryview 可以使用釋放 GIL 的路徑，同時可變緩衝區仍會重新取得 GIL，且相關行為有測試涵蓋。