Add thorough type annotations

This effectively bumps the required Python version to 3.7.x due to the
use of `from __future__ import annotations`, but in return we get static
type validation and potential for mypycification.

Author: elprans

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10.0-rc.1]
+        python-version: [3.7, 3.8, 3.9, 3.10.0-rc.1]
 
     env:
       PIP_DISABLE_PIP_VERSION_CHECK: 1

MANIFEST.in

@@ -0,0 +1,2 @@
+include LICENSE* README.rst
+include parsing/py.typed