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

首先查看 CPython PR #17757 和关联的 PR gh-148456，然后跟踪针对 memoryview 对象的 b"".join 处理逻辑。完成的标准是：不可变字节 memoryview 可以使用释放 GIL 的路径，同时可变缓冲区仍会重新获取 GIL，并且相关行为有测试覆盖。