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 Job ID Name Coverage
  Lint
passed #273708
black

00:00:26

passed #273710
check-manifest

00:00:25

passed #273709
flake8

00:00:24

passed #273711
mypy

00:01:31

 
  Tests
passed #273712
py3

00:05:17

 
  Release
passed #273713
image_build

00:01:42