math/py-piranha: Fix build with Python 3.8 and 3.9
- Adjust regexp pattern to make builds with Python 3.8/3.9 possible and remove Python 2 support from it while I'm here because the port is Python 3 only since r559531.
Excerpt from the log:
ninja: error: '/usr/local/lib/libpython3.8m.so', needed by 'pyranha/_core.so', missing and no known rule to make it
Approved by: portmgr (build fix)