pre-commit: Bump mypy to current latest version

* The older version didn't like `check_untyped_defs = True` in .mypy.ini.
* Trying `-repo: local` lead to an error about config options.
* We really need a way to have this track the non-pre-commit version of mypy.

.pre-commit-config.yaml

@@ -46,7 +46,7 @@ repos:
 # mypy - static type checking
 # mypy --follow-imports skip <file>
 -   repo: https://github.com/pre-commit/mirrors-mypy
-    rev: 'v0.931'
+    rev: 'v0.991'
     hooks:
       - id: mypy
         # verbose: true