fix first deprecation warnings

[syphar]

there is much more which can be made better, but this is a first step

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.18% :tada:

Comparison is base (ae8c6ff) 94.14% compared to head (0d8d874) 94.32%. Report is 5 commits behind head on master.

:exclamation: Current head 0d8d874 differs from pull request most recent head c987238. Consider uploading reports for the commit c987238 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #55 +/- ## ========================================== + Coverage 94.14% 94.32% +0.18% ========================================== Files 6 6 Lines 188 194 +6 ========================================== + Hits 177 183 +6 Misses 11 11 ``` | [Files Changed](https://app.codecov.io/gh/thermondo/pytest-translations/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=thermondo) | Coverage Δ | | |---|---|---| | [pytest\_translations/\_\_init\_\_.py](https://app.codecov.io/gh/thermondo/pytest-translations/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=thermondo#diff-cHl0ZXN0X3RyYW5zbGF0aW9ucy9fX2luaXRfXy5weQ==) | `100.00% <100.00%> (ø)` | | | [pytest\_translations/mo\_files.py](https://app.codecov.io/gh/thermondo/pytest-translations/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=thermondo#diff-cHl0ZXN0X3RyYW5zbGF0aW9ucy9tb19maWxlcy5weQ==) | `93.61% <100.00%> (+0.93%)` | :arrow_up: | | [pytest\_translations/po\_files.py](https://app.codecov.io/gh/thermondo/pytest-translations/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=thermondo#diff-cHl0ZXN0X3RyYW5zbGF0aW9ucy9wb19maWxlcy5weQ==) | `96.61% <100.00%> (ø)` | | | [pytest\_translations/po\_spelling.py](https://app.codecov.io/gh/thermondo/pytest-translations/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=thermondo#diff-cHl0ZXN0X3RyYW5zbGF0aW9ucy9wb19zcGVsbGluZy5weQ==) | `89.47% <100.00%> (ø)` | |

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[syphar]

@pcrock-thmdo the other things were general refactorings that we should do at some point, not python warnings.

Thanks for the review!