Deprecate is_pyqt5.

[anntzer]

The function won't really make sense anymore after the release of
PyQt6.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[anntzer]

the check is redundant with the try... except AttributeError.

[QuLogic]

Much like the Python 3 transition, do we want to check for >=5 instead? Otherwise, PyQt6 will use the PyQt4 code, but I assume the PyQt6 release will be somewhat closer to the 5 version than 4.

[anntzer]

sounds good.