Drop socknumbers workaround

[chrysn]

Most of the issues due to which there exists aiocoap.util.socknumbers have long been resolved.

One remains that is solved in python/cpython#118987 -- as that missed the Python 3.13 beta, chances are this issue will just stay open (and socknumbers.py unmodified) until we drop support for 3.13 (which will be some time).

[chrysn]

This has been reduced to what'll persist to around the 3.13 removal (or 3.12? not sure, might already be in the beta) in PR !16.