Fixed Python bugs + added unittests

[ColinKennedy]

addresses: #188

• Adds a section in the README.md on how to run unittests
• Python Exceptions are now grouped by type (previously it would list each duplicate)
• A function with yield should never include Returns: (it was accidentally doing this because of type-hints)

What this PR does not include

(From the original issue)

• typed *args / **kwargs not generating as expected

I don't know how to fix that one easily. It's still messed up.

[ColinKennedy]

CI is erroring on tests/neogen/lua_emmylua_spec.lua. Not sure why. The new Python tests are working though.

[danymat]

CI is erroring on tests/neogen/lua_emmylua_spec.lua. Not sure why. The new Python tests are working though.

Yup, I can't make it work. Seems like something is off with TS lua. Others seem fine though

I take a look to your PR and update you afterwards

[danymat]

Thanks for the job here ! The tests are a delight