libevent: remove Python dependency

Python is only used for one test, which is optional and is skipped if
Python is not detected. Additionally, Python 2 is required (Python 3
does not work), so remove the dependency to reduce the closure size for
a Python 3 as default Python future.

Additionally, update the meta.homepage and src to point at GitHub, which
is the libevent project home since 2015-10-07.