Fix dependencies (#81)

* Remove obsolete setuptools from install_requires

Now that pkg_resources are no longer used, there is no need to depend
on setuptools at runtime.

* Fix version-conditional dependency on importlib-metadata

Use version markers to depend on importlib-metadata correctly.  Explicit
conditions mean that wheels built with setup.py will either have the dep
or not depending on what Python version they're built with, rather than
what version they're installed on.

setup.py

@@ -5,10 +5,8 @@
 
 
 install_requires = [
-    'setuptools',
+    'importlib-metadata;python_version<"3.8"',
 ]
-if sys.version_info < (3, 8):
-    install_requires.append('importlib-metadata')
 package_excludes = ['tests*', 'docs*']
 packages = find_packages(exclude=package_excludes)