Support Python 3.6

It doesn't look like empyre depends on any new features or changes
in Python 3.7.

See also https://github.com/LiberTEM/LiberTEM/pull/851

setup.cfg

@@ -33,7 +33,7 @@ include_package_data = True
 package_dir =
     =src
 packages = find:
-python_requires = >=3.7
+python_requires = >=3.6
 setup_requires =
     setuptools
 install_requires =