Avoid implicitly depending on setuptools

[zware]

Currently there's an implicit dependency on pkg_resources from setuptools to get the version of the package. This makes the dependency explicit for Python 3.7 and avoids it entirely for 3.8+ which have importlib.metadata available.

[coveralls]

Coverage: 88.911% (+0.07%) from 88.84% when pulling 4543822cd52e3f169930528715d9f5501d0a8466 on zware:fix_implicit_setuptools_dependency into 4fa1f9dcc4ee811b35163ba629ddff0d38359cb7 on mosquito:master.

[zware]

Thanks!