py is a shortcut that launches the *latest* version of Python on the
machine. https://stackoverflow.com/a/50896577/632035
The build machines were updated to include python 3.9, so we were
doing everything with 3.9 instead of 3.8. pyproj doesn't have a binary
wheel for 3.9 on pypi yet, so we were falling back to building it from
source, which we aren't able to do, breaking the build.
