Add coverage reports for the python tests

[4thel00z]

Currently the python tests (to be implemented) in #8 are missing coverage information.

[XVilka]

We have Coveralls enabled for our account, thus it's possible to use it:

• https://github.com/codecov/example-python
• https://github.com/rizinorg/rizin/blob/dev/.github/workflows/ci.yml#L223