Move to python3

[mseri]

In a recent PR I added support for python3 (keeping python2 compatibility) in order to fix the CI, however this is getting hacky and it would be cleaner to just drop python2 support moving forward. Besides python2 is out of support.

I ping here @edwintorok and @vycastor as users of the library to discuss if this can be a problem for them. Any other user input will be welcome

[vycastor]

Hi! I don't use the python part of ocaml-rpc at all, so it's nothing of an issue for me.

[edwintorok]

According to https://pythonclock.org/ there are still 25 days left for Python2 :)
It is entirely understandable that upstream packages won't maintain Python2 support past its official EOL, doing so would be the responsibility of downstreams who (for some reason) are still relying on Python2 (like us).
FYI @MarkSymsCtx