Fix codecov upload reliability issues

[robrap]

Many PRs temporarily fail due to codecov upload failing in a flaky manner.

Here is an example failure: https://github.com/openedx/edx-drf-extensions/actions/runs/7182695502/attempts/2?pr=415

tests (ubuntu-20.04, 3.8, django42-drflatest)
Codecov: Failed to properly upload: The process '/home/runner/work/_actions/codecov/codecov-action/v3/dist/codecov' failed with exit code 255

Leading ideas to fix are:

1. Implement a retry, or
2. Drop codecov

If we decide to drop codecov (without replacing it), the implementation would be simple. However, a decision would need to be made and it needs to be understood where and who relies on codecov.

Another option is to fix this pain point, either temporarily (while a larger decision is being made), or more permanently. One (untested) solution might be https://github.com/Wandalen/wretry.action found in this Stackoverflow answer.