Fix Pytest using the wrong Python installation, add Pyright to legacy Python workflow, fix syntax errors on Python 3.5, remove Pyflakes

[jakub-nt]

Turns out that during legacy Python test workflows, Pytest did not use the virtual environment's legacy Python version:

Run pytest
============================= test session starts ==============================
platform linux -- Python 3.12.3, pytest-8.4.1, pluggy-1.6.0
{...}