feat(ci): remove `codecov` step from `tox` since performed in GA

[StephenSorriaux]

Why is this needed?

There is no need to perform the non-working codecov step via tox since we now have a Github Actions step that performs it.

Proposed Changes

• update tox.ini and remove codecov related stuff
• update GA testing.yml to remove codecov from matrix

Does this PR introduce any breaking change?

Nah.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.11 :tada:

Comparison is base (6dbf606) 96.28% compared to head (10daa15) 96.39%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #719 +/- ## ========================================== + Coverage 96.28% 96.39% +0.11% ========================================== Files 27 27 Lines 3554 3554 ========================================== + Hits 3422 3426 +4 + Misses 132 128 -4 ``` [see 4 files with indirect coverage changes](https://codecov.io/gh/python-zk/kazoo/pull/719/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-zk) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-zk). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-zk)

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[StephenSorriaux]

+1

To run coverage locally, someone doesn't need to specify anything beyond coverage, right? No extra env vars / flags? Or do they need to specify coverage -e ZOOKEPER_VERSION?

Locally, if you want to use coverage you would just need to perform coverage xml, yes. FWIW coverage is "integrated" with pytest, in the tox.ini configuration (the pytest-cov dep + the --cov and --cov-report flags).