Fix LICENSE file not to be installed into site-packages

[mgorny]

The include directory includes the LICENSE files both in sdist and wheel archives by default, effectively causing it to be installed straight into site-packages, i.e.:

/usr/lib/python3.10/site-packages/LICENSE

Limit it to sdist format to prevent that.

Fixes #92

[codecov[bot]]

Codecov Report

Base: 98.24% // Head: 98.24% // No change to project coverage :thumbsup:

Coverage data is based on head (359c0a2) compared to base (046f2bc). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #97 +/- ## ======================================= Coverage 98.24% 98.24% ======================================= Files 8 8 Lines 399 399 Branches 36 36 ======================================= Hits 392 392 Misses 4 4 Partials 3 3 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Vu+Tran). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Vu+Tran)

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[mgorny]

Thanks!