Use Python 3.10 as the default Python version for new apps (#1296)

Previously if a new app did not specify a Python version via a
`runtime.txt` or the appropriate `Pipfile` config option, then the
buildpack would default to the latest Python 3.9 release (currently
Python 3.9.11).

Now the buildpack will use the latest Python 3.10 release (currently
Python 3.10.3).

This does not affect:
* apps that specify an explicit Python version
* existing apps that have already had a build using the previous default
  version (since that version is remembered from one build to the next,
  via the "sticky versions" feature).

Use-cases involving new apps that aren't compatible with Python 3.10
will now need to specify Python 3.9 explicitly:
https://devcenter.heroku.com/articles/python-support#specifying-a-python-version

GUS-W-10881777.