docs: cleanup (#567)

* fix: support docs/tests extras, simpler RtD install

Use oldest-supported-numpy from SciPy in example

* chore: bump to ubuntu-20.04 images

* fix: move extras to setup so we can do combinations

.pre-commit-config.yaml

@@ -20,7 +20,7 @@ repos:
   rev: 20.8b1
   hooks:
   - id: black
-    files: ^bin/update_pythons.py$
+    files: ^bin/update_pythons.py|setup.py$
 
 - repo: https://github.com/pre-commit/mirrors-mypy
   rev: v0.800