Ditch Codecov

[pauloxnet]

As suggested by @adamchainz in this comment it would be useful for the project to ditch Codecov https://hynek.me/articles/ditch-codecov-python/

[uhurusurfa]

Although it now works with the updates, it seems to be unreliable and have just had to rerun a python 3.9 test which failed to upload to codecov but passed on the re-run I triggered a couple minutes later so perhaps we should ditch codecov entirely