Keep directories holding deeper files in a recursive MemoryFileSystem.rm with maxdepth

[raashish1601]

MemoryFileSystem.rm(path, recursive=True, maxdepth=...) raises when a directory at the last expanded level still holds files below maxdepth:

m = fsspec.filesystem("memory")
m.mkdir("/src/sub/deeper")
m.pipe({"/src/top.txt": b"", "/src/sub/mid.txt": b"", "/src/sub/deeper/low.txt": b""})
m.rm("/src", recursive=True, maxdepth=2)
# OSError: Directory not empty: '/src/sub/deeper'

Without the mkdir (implicit directories) it raises FileNotFoundError: /src/sub/deeper instead.

expand_path stops at maxdepth, so /src/sub/deeper is in the list but low.txt is not, and rmdir refuses the non-empty directory. Before raising it has already deleted top.txt and mid.txt.

#2238 made LocalFileSystem.rm keep a directory that still holds deeper files in this case. This does the same for the memory filesystem: when maxdepth is given, a directory that is not empty after the files above it are removed is kept. Without maxdepth, or for a non-recursive rm of a non-empty directory, nothing changes.

Tests: added test_rm_recursive_maxdepth_keeps_deeper_files to test_memory.py (with implicit and explicit directories), mirroring the local test. Both cases fail on master and pass here. test_memory.py and the memory abstract tests pass, and ruff/codespell are clean. Added a changelog entry.

[raashish1601]

The pytest-win failures are the three test_github.py tests hitting the GitHub API rate limit (403 rate limit exceeded); they don't touch the memory filesystem. A re-run should clear them.