Use a lock file to avoid exceptions due to concurrenct symlink creation

[stefanberger]

We have seen exceptions being raised from _update_root_symlink() on the level of the sigstore-python library when multiple concurrent threads were creating symlinks in this function with the same symlink name (in a test environment running tests concurrently). To avoid this issue, have each thread open a lock file and create an exclusive lock on it to serialize the access to the removal and creation of the symlink.

The reproducer for this issue, that should be run in 2 or more python interpreters concurrently, looks like this:

from sigstore import sign
while True:
    sign.TrustedRoot.production()

Use fcntl.lockf-based locking for Linux and Mac and a different implementation on Windows. The source originally comes from a discussion on stockoverflow (link below).

Resolves: #2836
Link: https://stackoverflow.com/questions/489861/locking-a-file-in-python

Description of the changes being introduced by the pull request:

Fixes #2836