Fix PyQt5 version check

[vkbo]

Summary:

Seems QT_VERSION_STR has been imported from a different location than what's listed in the PyQt5 documentation. It may have moved at some point, or the import from PyQt5.Qt may have been a bleed through that is no longer there.

This PR imports it from PyQt5.QtCore instead, as the PyQt5 docs state. The PR also replaces the 10 base integer version with the hex version provided by PyQt5. There is only one version check left in the main code anyway.

Related Issue(s):

Resolves #1324

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed