Fix inclusion of tests in source dist

[codecov[bot]]

Codecov Report

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

Project coverage is 95.24%. Comparing base (8e6dbea) to head (96c5161).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #188 +/- ## ========================================== + Coverage 95.23% 95.24% +0.01% ========================================== Files 20 20 Lines 1405 1409 +4 ========================================== + Hits 1338 1342 +4 Misses 67 67 ```

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