nslink.NetNS is possibly not safe to use in multithreaded applications

[philipp-karcher]

We have been dealing with very mysterious deadlocks in a multithreaded application, and I believe that nslink.NetNS is the culprit. It calls os.fork in its constructor, which is unsafe when mixed with threads (and for this reason deprecated in Python 3.12):

https://docs.python.org/3/library/os.html#os.fork
https://discuss.python.org/t/concerns-regarding-deprecation-of-fork-with-alive-threads/33555