Fix codecov upload reliability issues

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.

Start with the linked GitHub Actions failure and locate the workflow step that invokes the Codecov action. Compare the retry and removal options, including who relies on Codecov, then confirm that the chosen change prevents temporary Codecov failures from failing pull-request checks.