Everywhere: on CI, in tests, in docs. Also, we replace bare pip/pip3 and pytest calls with python -m pip and python -m pytest respectively, to make sure we always use the same python executable everywhere.

This fixes #326 (failing Windows tests on CI), because the `python3` executable causes issues (it is not properly copied into the venv directory by venv, which is a CPython bug, see python/cpython#87915), while the python executable does work; see e.g. here https://stackoverflow.com/questions/61669873/python-venv-env-fails-winerror-2-the-system-cannot-find-the-file-specified. `python` should be the same executable under the hood as `python3`, if the setup-python action works as expected, at least.
It also allows us to remove the IS_WINDOWS_CI special case.

Failed attempts at fixing #326 included:
- Setting the shells (assuming something went wrong with environment
  variables)
- Using full paths for both the executable and the venv directory in the
  template test suite.