devel/libevent: Don't force Python 2.7

There is no need to force the use of python 2.7 in libevent, it has been
updated to work with python 3 in this upstream commit:
https://github.com/libevent/libevent/commit/8b0aa7b36a3250fad4953f194c8a94ab25032583
Bump portrevision to force rebuild

PR: 246465
Reported by: gahr@