Exclude tests packages from distribution

[shawnz]

Fixes #618

This change adds tests and tests.* to the exclude parameter of find_packages. Previously, installing guidance would also create an unexpected tests subdirectory in the user's site-packages directory, which can cause issues if the user has their own package also named tests.

I validated this change with the following:

python3 -m venv .venv
source .venv/bin/activate
pip install .[test]

and verified that .venv/lib/python3.10/site-packages did not contain a tests subdirectory. Furthermore, I verified that running pytest still works as expected after the change

[Harsha-Nori]

I think this seems totally reasonable. Thanks for digging into and patching this up @shawnz!