Refactor and unit test Python version from classifier code

There are service tests which cover the functionality, however you can't tell what they are doing from looking at the code. See https://github.com/badges/shields/pull/3014#discussion_r for an example that conveys the intent.

All the remaining classifier functionality could be extracted to helper functions, as we've done with with `getLicenses` and `getPackageFormats` in `pypi-helpers.js`.

Review the existing service tests and pypi-helpers.js, then read the linked pull request discussion for an example of the intended test clarity. Done means the remaining classifier functionality is extracted into helper functions and covered by unit tests that make each behavior's intent clear.

Written by the indexing model from the issue text.