[IO] remove outdated register_cp65001() function
[IO] remove outdated register_cp65001() function

cp65001 is an alias for utf-8 since Python 3.8
and was added to the codecs table in Python 3.3 already

https://docs.python.org/3/library/codecs.html
https://docs.python.org/3.7/library/codecs.html

Change-Id: I35277ca3d4a018cc4673c5ac84a8e89e4026cfd0

wikimedia/pywikibot-core,wikimedia/pywikibot-core