Add a version requirement for setuptools

[xhochy]

Installing PyScaffold warns about an outdated version of setuptools. Instead it should just depend on a newer version.

your setuptools is too old (<12)
setuptools_scm functionality is degraded

[FlorianWilhelm]

From commit 1ef0a90 onwards, Pyscaffold as well as projects created with it will require at least setuptools>=9.0.