Remove standalone pytest run_tests.py bundle Newer pytest doesn't support generating stand alone testing bundles. Python 3.9 generates errors running the current run_tests.py. Changed doc to use pythonX -m pytest test/ and some light directions on how to install pytest. Thanks to jerrykan for recommendations on how to do the doc updates.

[run]

# ignore the dist utils. They are not part of the code base
# we are going to test. They are used by maintainers only.
omit = roundup/dist/*