Fix CI Python3 breakage

For some reason `brew install python3` is failing in GitHub Action's
macos-13 image due to stale 2to3/etc links. Simply remove the install
step. We no longer need it as we have stable ABI now and we don't need
latest version.

Also, fix Python 2's installation link using an outdated version for
some reason.

.github/workflows/ci-macvim.yaml

@@ -126,7 +126,10 @@ jobs:
       - name: Install packages
         if: matrix.publish
         run: |
-          brew install python3
+          # We no longer need to install/update Python 3, as it's guaranteed to
+          # be installed on runners. Since we use stable ABI, the exact version
+          # on CI does not matter.
+
           brew install ruby
           brew install lua
 
@@ -146,8 +149,8 @@ jobs:
           #
           # This will be removed in the future as Python2 has been completely
           # unsupported for years.
-          curl https://www.python.org/ftp/python/2.7.16/python-2.7.16-macosx10.9.pkg -o ~/Downloads/python-2.7.16-macosx10.9.pkg
-          sudo installer -pkg ~/Downloads/python-2.7.16-macosx10.9.pkg -target /
+          curl https://www.python.org/ftp/python/2.7.18/python-2.7.18-macosx10.9.pkg -o ~/Downloads/python-2.7.18-macosx10.9.pkg
+          sudo installer -pkg ~/Downloads/python-2.7.18-macosx10.9.pkg -target /
 
       # All set up steps are done. Build and test MacVim below.