Update code/test to work with latest Python 3.12 and add CI

[phillip-stephens]

Wanted to remove the dependency of using Python 2 in ZGrab, so updating zschema to be more current.

• Switch from the deprecated nose to pytest, swapping out equivalent functions so it compiles
• Added GH CI to run pytest on all PR's
• Follow current best-practices for using SetupTools, eschewing setup.py for a pyproject.toml file
• Ran black . to lint

Testing

Tested with python 3.12, ran pytest and all tests pass with no warnings

~/zschema on  master! ⌚ 16:09:14
$ pytest
================================== test session starts ==================================
platform darwin -- Python 3.12.3, pytest-8.3.2, pluggy-1.5.0
rootdir: /Users/phillip/zschema
configfile: pyproject.toml
plugins: xdist-3.6.1
collected 48 items

zschema/test_zschema.py ................................................          [100%]

================================== 48 passed in 0.05s ===================================