devel/py-pygit2: Fix RUN_DEPENDS

• cached-property is only required for Python 3.7 and below
• six is not required
• Update version requirement of *_DEPENDS
• Bump PORTREVISION for dependency change

from setup.py:
install_requires = ['cffi>=1.4.0']
if sys.version_info < (3, 8):

install_requires.append('cached-property')

...

setup_requires=['cffi>=1.4.0'],

from CHANGELOG.rst:

• Don't require cached-property with Python 3.8 or later

...

• Drop Python 2.7 and 3.4 support, six no longer required

Approved by: portmgr (blanket)
With hat: python