Drop Python2 support

[sobolevn]

Right now almost no users are on Python2: https://pypistats.org/packages/eradicate

And it is long gone. I propose to drop it, because even actions-python won't support it soon.

Python2 users can continue using 2.2 and 2.3

[sobolevn]

Plus, I've made an official announcement about this in https://github.com/wemake-services/eradicate/releases/tag/2.3.0