py: use setup.cfg to configure setuptools

py: use setup.cfg to configure setuptools

Setuptools has had support for declarative configuration for several
years.  To quote their documentation:

  Setuptools allows using configuration files (usually setup.cfg) to
  define a package’s metadata and other options that are normally
  supplied to the setup() function (declarative config).

  This approach not only allows automation scenarios but also reduces
  boilerplate code in some cases.

Additionally, this allows us to introduce support for PEP-517-compatible
build-systems.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>