devel/py-backports.*: resolve conflicts with devel/py-backports

devel/py-backports does basically nothing except to install the
/usr/local/lib/python3.*/site-packages/backports/__init__.py file.
Several other py-backports.* ports also installed the same file, with
identical contents. That created runtime conflicts that prevented
certain ports from being installed, including sysutils/py-salt.

Fix the conflicts by patching the offending ports to stop installing
that file, and instead depend on devel/py-backports.

PR: 297696
PR: 297697
Reviewed by: krion
Sponsored by: ConnectWise
Inspired by: 56a719e6b67 (svn r492366)