Revisit PyQt version(s) requirements

[lbianchi-lbl]

Motivation

• Currently, we're using pyqt==5.13 in setup.py
• However, this has shortcomings
  • AFAIK 5.13 is not a "super-stable"/LTS (long-term support) version for (Py)Qt 5 such as 5.12 and 5.15
  • To be able to have a Conda package for FOQUS, it has to be compatible with the available versions

Known constraints

• pytest-qt only works with Qt 5.11+
• AFAIK, PyQt 5.12.3 is the latest version available to install with Conda (through the conda-forge channel)
• PyQt 5.13 is not available for Windows and Python 3.9, leading to installation errors such as ERROR: Could not find a version that satisfies the requirement PyQt5==5.13 (from ccsi-foqus) (from versions: 5.12.3, 5.14.0, 5.14.1, 5.14.2, 5.15.0, 5.15.1, 5.15.2, 5.15.3, 5.15.4, 5.15.5, 5.15.6)

[ksbeattie]

Partially addressed in PR #1014