devel/py-bitstring: Update to 3.1.7

• Update to 3.1.7
• The sdist on PyPI changed from .zip to .tar.gz
• Add NO_ARCH

Reviewed by: koobs, sbz
Approved by: koobs (ports, mentor, implicit), sbz (maintainer)
Differential Revision: https://reviews.freebsd.org/D30651
MFH: No (feature release)