Python: add support for pyproject.toml (pep621) and perhaps setup.cfg specifications of dependencies

[jenshnielsen]

pep621 specifies a standardized way of specifying dependencies in a declarative way. It would be nice if this could be directly supported. While in principle the current way of executing setup.py via distutils does pick this up there are several issues with it. See comment in #108 and #244

This is now officially supported in setuptools and is expected to be used in many more python packages going forward

It would also be nice to directly support dependencies in setup.cfg which is setuptools old but still widely used declarative config format

AB#2122796

[JamieMagee]

Multiple users are now requesting this. It should bump the priority.