GitHub Actions: Replace nose with pytest

As demonstrated in #40, The tool nose2pytest finds no tests to convert.

So try testing with pytest... With output like https://github.com/cclauss/PACKMAN/actions

============================= test session starts ==============================
platform linux -- Python 3.9.16, pytest-7.2.2, pluggy-1.0.0
rootdir: /home/runner/work/PACKMAN/PACKMAN
collected 15 items

packman/tests/anm/test_anm.py ..                                         [ 13%]
packman/tests/dci/test_dci.py .                                          [ 20%]
packman/tests/entropy/test_entropy.py .                                  [ 26%]
packman/tests/geometry/test_geometry.py ..                               [ 40%]
packman/tests/gnm/test_gnm.py .                                          [ 46%]
packman/tests/molecule/test_molecule.py ........                         [100%]

=============================== warnings summary ===============================
packman/tests/geometry/test_geometry.py::TestGeometry::test_AlphaShape
  /home/runner/work/PACKMAN/PACKMAN/packman/tests/geometry/test_geometry.py:22: DeprecationWarning: Delaunay attribute 'vertices' is deprecated in favour of 'simplices' and will be removed in Scipy 1.11.0.
    self.assertIsInstance( geometry.AlphaShape( [j for i in self.mol[0].get_backbone() for j in i], 4 )[0][0], molecule.Atom )

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================== 15 passed, 1 warning in 6.09s =========================

Added the pytest warning as issue:

Pranavkhade / PACKMAN

GitHub Actions: Replace nose with pytest #39

41