Initial typing support

This enables type checking in CI and marks the project as typed.

requirements/typing.txt

@@ -0,0 +1,14 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile requirements/typing.in
+#
+mypy-extensions==0.4.3
+    # via mypy
+mypy==0.812
+    # via -r requirements/typing.in
+typed-ast==1.4.3
+    # via mypy
+typing-extensions==3.7.4.3
+    # via mypy