devel/py-setuptools_scm: Turn py-packaging into a soft dependency

• Since the 6.3.0 release, devel/py-packaging has been defined in the "setup.cfg" as an unconditional dependency. However this leads to problems if py-packaging is uninstalled but py-setuptools_scm persists, then the missing port can no longer be built. [1]
  
  Although py-setuptools_scm has a fallback to devel/py-setuptools when py-packaging is absent this fallback code is not executed. This is because py-setuptools, which checks the dependencies of py-setuptools_scm upon invocation and terminates as soon as these are not met.
  
  Fix this issue by converting py-packaging into an optional dependency for py-setuptools_scm so that the fallback code is really used if py-packaging is absent. But keep the latter one in the RUN_DEPENDS in order to continue to reflect the actual dependencies.

• Also add py-packaging to BUILD_DEPENDS as defined in "pyproject.toml" although it's not strictly required to build the port. [2]

• Bump PORTREVISION due package change.

PR: 259981
Reported by: many [1], koobs [2]
Tested by: se, myself