Add unversioned brew python links to PATH

The `python` Homebrew package installs unversioned links to Python 3
executables in `/usr/local/opt/python/libexec/bin`. Add this directory
to the `PATH` environment variable when it's present.

zshenv

@@ -22,6 +22,8 @@ INFOPATH=$HOME/.local/share/info:$INFOPATH
 
 # Add ccache to the PATH and use XDG base dir paths
 if [ `uname` = Darwin ]; then
+  [ -d /usr/local/opt/python/libexec/bin ] && \
+    PATH=/usr/local/opt/python/libexec/bin:$PATH
   [ -f /usr/local/bin/ccache ] && \
     PATH=/usr/local/opt/ccache/libexec:$PATH
 elif [ -f /usr/bin/ccache ]; then