feat(mypy): Add basic configuration file for Mypy

- add basic mypy config
- update mypy command inside tox config
- set hg to ignore mypy_cache folder
- add just enough type inside schema.py to please mypy
6 jobs for !37 with topic/default/please-mypy in 8 minutes and 39 seconds (queued for 8 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed black #273708

00:00:26

passed check-manifest #273710

00:00:25

passed flake8 #273709

00:00:24

passed mypy #273711

00:01:31

 
  Tests
passed py3 #273712

00:05:17

 
  Release
passed image_build #273713

00:01:42