x11-toolkits/py-kivy: resurrect the port and update to version 2.3.0

Refresh dependencies and convert to USES=+pytest (this also allows to
drop `do-test' target). Unfortunately, not all tests currently pass:

15 failed, 788 passed, 42 skipped, 1 xfailed, 8 warnings, 70 errors

Requested by: Sergey V. Dyatko