Python 3.5 compatibility

After all, we don't use any 3.6-specific feature, so should just support
all major python 3 versions.

setup.py

@@ -28,7 +28,7 @@ setup(
     long_description=readme,
     packages=["tutor"],
     include_package_data=True,
-    python_requires=">=3.6",
+    python_requires=">=3.5",
     install_requires=[
         "appdirs",
         "click>=7.0",
@@ -48,6 +48,7 @@ setup(
         "License :: OSI Approved :: GNU Affero General Public License v3",
         "Operating System :: OS Independent",
         "Programming Language :: Python",
+        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
     ],