add tests

I have now updatedLib/test/test_socketserver.py to include the appropriate tests. Turns out it was defining its ownForkingUnix{Stream,Datagram}Server within the test file exactly as we are adding tosocketserver, who would've guessed hahaha.

and docs

The current docs already describe in detail theThreadingMixIn andForkingMixIn, when to use them etc. The only thing that really needed adding was the mentions of the new servers at the'These classes are pre-defined using the mix-in classes' section. The existing documentation only mentions the{Forking,Threading}{TCP,UDP}Server classes once in this section, and the new docs in this PR are consistent with this; the documentation seems enough.

Thanks for the intermediary commits. The PR is now up-to-date again with main and is conflict free.