Hey, thanks for the report.
You uncovered a rather embarrassing oversight — I poured a lot of attention into the wheels but didn't test the sdist (?), so the sdist is missing a header and I didn't even notice. This is fixed in v1.0.1 I just released.
Now it works on my rpi 3B running raspbian buster with Python 3.7.3 (requires `libgmp-dev`, needless to say).