Fix failing tests

[pjbull]

Two fixes to get a passing test suite

• Bound coverage to work with pytest-cov
• Install if no previous version

Fixes #111 Fixes #110 Fixes #109

[github-actions[bot]]

🚀 Deployed on https://deploy-preview-112--zen-roentgen-1e5a72.netlify.app

[codecov-commenter]

Codecov Report

Merging #112 (28f6828) into master (e492103) will not change coverage. The diff coverage is 100.0%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #112 +/- ## ====================================== Coverage 98.5% 98.5% ====================================== Files 8 8 Lines 425 425 ====================================== Hits 419 419 Misses 6 6 ``` | [Impacted Files](https://codecov.io/gh/drivendataorg/nbautoexport/pull/112?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=drivendataorg) | Coverage Δ | | |---|---|---| | [nbautoexport/jupyter\_config.py](https://codecov.io/gh/drivendataorg/nbautoexport/pull/112/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=drivendataorg#diff-bmJhdXRvZXhwb3J0L2p1cHl0ZXJfY29uZmlnLnB5) | `97.1% <100.0%> (ø)` | |