feat(provider)!: remove support for python2 and python3.[3-5]

These versions of python has reached End-of-life. getting rid
of python2 support removes a lot of logic to support two
incompatible python versions in the same version.

src/nvim/testdir/test_python3.vim

@@ -69,6 +69,7 @@ func Test_vim_function()
 endfunc
 
 func Test_skipped_python3_command_does_not_affect_pyxversion()
+  throw 'skipped: Nvim hardcodes pyxversion=3'
   set pyxversion=0
   if 0
     python3 import vim