Bump minimum python version to 3.7

Python lower than 3.7 is EOL, we're supporting up to what's currently
released, namely 3.11.

setup.cfg

@@ -14,14 +14,15 @@ classifiers =
     Development Status :: 3 - Alpha
     Intended Audience :: Developers
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
+    Programming Language :: Python :: 3.11
     License :: OSI Approved :: Apache Software License
 
 [options]
-python_requires = >= 3.5
+python_requires = >= 3.7
 packages = clintermission
 setup_requires =
     setuptools > 30.4.0