Closes#3881.
On Windows, _rebuild_lock falls back to yield True when fcntl cannot be
imported, leaving concurrent rebuilds completely unserialized and causing
file collisions on .graph.tmp.json.
Implement Windows file locking using Python's standard library msvcrt module.
Lock a byte range at offset 4096 so external readers can still read the owning
PID at byte 0 without hitting Windows mandatory lock PermissionErrors.
Un-skip the 4 rebuild lock tests and 3 pending queue tests on Windows, and
add test_rebuild_lock_blocks_concurrent_process.
(cherry picked from commit d8cd456650)
harshaygadekar
e4a6f5098e