* Detect list mutation during sort even when list length is unchanged
* Use mutation counter instead of capacity check for sort mutation detection
The capacity heuristic missed mutations when `clear()` reset capacity to
0 via `mem::take`. An AtomicU32 counter on PyList, incremented in
`borrow_vec_mut()`, reliably detects all mutations during sort.
* Hold write guard during sort mutation counter reads
* Fix list mutation counter race in `borrow_vec_mut`
* Downgraded skips in tests
* Fixed failing tests
* Fixed test_ftplib + test_socket + test_ssl + test_threaded_import failures
* Removed comments about which tests are run in which environment
* Addressed PR comments
* Annotated skips on failing tests
* Removed unneeded tests
* Removed unneeded sys import from test_ftplib
* Added annotation to test_ftplib
* Readded skipIf to test_cleanup_with_symlink_modes with a more general ENV_POLLUTING_TESTS_WINDOWS
* Addressed PR comments
* Made changes to minimize diff in PR
* Apply suggestion from @youknowone
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Co-authored-by: Jeong, YunWon <69878+youknowone@users.noreply.github.com>
