Remove codecov dependency

[johannes-mueller]

From yesterday on, the codecov package is no longer available on pypi. That makes pip install testbook[dev] fail as the dependency to codecov cannot be resolved.

https://about.codecov.io/blog/message-regarding-the-pypi-package/

Workaround is to use

pip install testbook ipykernel

instead.

[rohitsanj]

Looks like they re-instated the coverage package to PyPI on Apr 17, 2023. Either way, I've removed codecov as dev dependency in #158.

[johannes-mueller]

Thanks a lot.