Use __all__

[Zethson]

Closes #583

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 62.22%. Comparing base (df4b8ab) to head (9cbb02e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #588 +/- ## ========================================== + Coverage 62.19% 62.22% +0.03% ========================================== Files 43 43 Lines 5295 5300 +5 ========================================== + Hits 3293 3298 +5 Misses 2002 2002 ``` | [Files](https://app.codecov.io/gh/theislab/pertpy/pull/588?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab) | Coverage Δ | | |---|---|---| | [pertpy/data/\_\_init\_\_.py](https://app.codecov.io/gh/theislab/pertpy/pull/588?src=pr&el=tree&filepath=pertpy%2Fdata%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L2RhdGEvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [pertpy/metadata/\_\_init\_\_.py](https://app.codecov.io/gh/theislab/pertpy/pull/588?src=pr&el=tree&filepath=pertpy%2Fmetadata%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L21ldGFkYXRhL19faW5pdF9fLnB5) | `100.00% <100.00%> (ø)` | | | [pertpy/plot/\_\_init\_\_.py](https://app.codecov.io/gh/theislab/pertpy/pull/588?src=pr&el=tree&filepath=pertpy%2Fplot%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L3Bsb3QvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [pertpy/preprocessing/\_\_init\_\_.py](https://app.codecov.io/gh/theislab/pertpy/pull/588?src=pr&el=tree&filepath=pertpy%2Fpreprocessing%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L3ByZXByb2Nlc3NpbmcvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [pertpy/tools/\_\_init\_\_.py](https://app.codecov.io/gh/theislab/pertpy/pull/588?src=pr&el=tree&filepath=pertpy%2Ftools%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L3Rvb2xzL19faW5pdF9fLnB5) | `100.00% <100.00%> (ø)` | |