Move devel/py-gflags to devel/py-python-gflags and update to 3.1.0

• Add LICENSE_FILE
• Add NO_ARCH
• Relax USES=python
• Allow concurrent installation (USE_PYTHON=concurrent)
• Remove PROJECTHOST
• Update WWW

Changes: https://github.com/google/python-gflags/releases
PR: 215614
Differential Revision: https://reviews.FreeBSD.org/D8926
Submitted by: sunpoet (myself)
Approved by: maintainer (timeout, 14 days)