Integrate with Codecov

[pvk-developer]

• Integrate with Codecov to make sure all the changes from PR improve the code coverage of tests.
• Update the readme with the new badge.

[01110011011101010110010001101111]

Seems fairly straightforward, would be happy to help out, but requires being a maintainer of the repository: https://docs.codecov.com/docs/code-coverage-with-python