Missing `LICENSE.txt` in 0.20.2

[mgorny]

Describe the bug

It seems that some recent packaging changes have caused the LICENSE.txt to disappear from the source distribution.

To Reproduce

Download source distribution from PyPI and look inside.

Expected behavior

LICENSE.txt with verbatim LGPL text.

Screenshots

n/a

Additional context

While working on the source distribution, could you please also include tests for downstream packaging purposes?

[nightcityblade]

Hi, I'd like to work on this. I'll submit a focused PR shortly.