Vim either py3 or py2 (issue #31337)

fixes for Vim and MacVim, as discussed in issue #31337

Note: I am not sure I understand why the changes I made on the new branch (`vim-either-py3-or-py2`) are not showing up here.
The pull request should change the formulas for Vim and MacVim, but all stuff listed below seems to be on quite different files.
If I locally do `git diff vim-either-py3-or-py2..master`, I do see the right changes (only) on the two formula files between my local `master` and `vim-either-py3-or-py2` branch, but I get entirely different stuff here on GitHub.
I hope this PR is right, as it is impossible to open the formula files here on GitHub (it only shows 1000 files/directory), otherwise please let me know how to fix it.