devel/py-icalendar: Allow build with Python 3.x

Allow building with Python 3.x (Supported since 3.6)

While I'm here,

• Add LICENSE_FILE

PR: 204935
Submitted by: Christian Geier <catcher+freebsdbugzilla lostpackets de>
Approved by: maintainer <jun-g daemonfreaks com>
MFH: 2015Q4