vendor pymysql byte/int utils

[zzzeek]

PyMySQL 1.0.0 has removed the util.py module as well as some utilities from connections.py; vendor the small set of very short functions locally and open up requirements to support PyMySQL 1.0.0

[Nothing4You]

hi,

thanks for the PR.

could you please reduce this to just vendoring pack_int24 and lenenc_int?
they should also be prefixed with underscores, as we do not want to add them to our public API.
the other methods should not be needed anymore and they should get removed in #643.

this change alone will still be compatible with PyMySQL <1.0.0, so if you remove bumping the PyMySQL version it will be compatible independently from merging #643.

[zzzeek]

I wont have time to refine this patch in the ways you seek so feel free to take over and/or close.

[Nothing4You]

no worries, thank you anyways.
I'll close this in favor of #643.