Unlike python of Homebrew, the python.org version comes bundled with
a recent Tk that is much less buggy than macOS stock Tk.
py2app does not seem to work with Pillow's pypi wheels.
The one used by default seems to be 5.18, which while not _very_ old (released 2013 or 2014,
depending on which patch version) seems a likely cause of the test failures I'm seeing in
CI but am unable to reproduce in my Linux environment (where 5.24.3 is the oldest version
available).
