fix(makefile): support python 3.10

[branchvincent]

This fixes a bug detecting the current Python version, noticed when building with Python 3.10 in Homebrew/homebrew-core#90708

Before:

$ make -C third-party/chpl-venv test-venv PYTHON=python3.10
python3 version 3.5 or later is required to install chpldoc and start_test dependencies. See https://www.python.org/

After:

$ make -C third-party/chpl-venv test-venv PYTHON=python3.10
...
Successfully installed ...

[bradcray]

Pinging @mppf on this since it's his code that's changed and we tend not to be notified when new PRs are opened.

[mppf]

@branchvincent - thanks very much for this PR. It looks like a good fix to me. I'd just like to do a test run with it before merging.

[mppf]

I've verified make test-venv fails with the expected error with Python 3.4.6 but succeeds with Python 3.6.12. I also checked that the Python snippet being run works on Python 2.7. Besides that I will do our usual pre-merge testing.

[mppf]

Passes full local testing.

[bradcray]

Thanks @branchvincent and @mppf!

[branchvincent]

@bradcray @mppf thank you for the quick review/merge!