devel/py-setuptools_scm: Update to 5.0.2

• Use this release as an intermediate step before the port is updated to the 6.x release, because it contains mostly bugfixes and is also required for the 2021Q4 branch.
  
  The latter is necessary in order to be able to build certain ports successfully if devel/py-setuptools_scm is already present in the build environment. For instance, devel/py-platformdirs, which can be built without it but if devel/py-setuptools is already installed, it requires the 5.x release of it for build. [1]

• Also adjust the description of the TOML option slightly with an appropriate warning to avoid build issues related to parsing "pyproject.toml" files during setup. [2]

Changelog since 4.1.2:

https://github.com/pypa/setuptools_scm/blob/v5.0.2/CHANGELOG.rst

PR: 259381 [2], 259420 (both related)
MFH: 2021Q4 (after 1 week) [1]