Declare supported Python versions in dist meta

Resolvelib is not maintained for Python prior to 3.7.
It was pointed out in sarugaku#116.

Resolves sarugaku#122

setup.cfg

@@ -26,6 +26,7 @@ package_dir =
 packages = find:
 include_package_data = true
 zip_safe = false
+python_requires = >=2.7, !=3.0, !=3.1, !=3.2, !=3.3, !=3.4, !=3.5, !=3.6
 
 [options.packages.find]
 where = src