Improve overall api usage

[pietzschke]

Checklist for your PR

• [ ] please start with a draft PR and mark it as ready for review when you are confident that your implementation works as expected.
• [ ] add a changelog entry in CHANGELOG.md with a link to this PR.
• [ ] make sure that all tests pass and add new test functions if your code is not covered by existing tests. Please use mock tests when network requests are required.
• [ ] let someone review & approve your code, make changes where necessary.

Hints

• name your branches accordingly: use the prefixes bugfix/, feature/, or release/ and then a short title for your PR in kebab-case (lowercase words separated with dashes), e.g. feature/my-awesome-feature.
• Please make sure that pre-commit hooks are enabled on your system to avoid common mistakes before you make a commit.
• feel free to delete this template and replace it with your own description of the PR.

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b5ead78) 87.31% compared to head (e7d0c96) 87.40%. Report is 2 commits behind head on master.

:exclamation: Current head e7d0c96 differs from pull request most recent head a5bff98. Consider uploading reports for the commit a5bff98 to get more accurate results

Files	Patch %	Lines
src/personio_py/mapping.py	86.66%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #37 +/- ## ========================================== + Coverage 87.31% 87.40% +0.09% ========================================== Files 7 7 Lines 946 961 +15 ========================================== + Hits 826 840 +14 - Misses 120 121 +1 ```

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