Python: drop python.majorVersion

Drop `python.majorVersion`. For Python language version, use `python.pythonVersion`.
For implementation version, use `python.sourceVersion`.

Some expressions were broken. Those that were identified were fixed.

fixup major

pkgs/development/interpreters/python/default.nix

@@ -39,8 +39,6 @@ with pkgs;
         pythonAtLeast = lib.versionAtLeast pythonVersion;
         pythonOlder = lib.versionOlder pythonVersion;
         inherit pythonForBuild;
-
-        majorVersion = pythonVersion; # Backwards compatibility. Should be dropped.
   };
 
 in {