specify revision for mypy

.pre-commit-config.yaml

@@ -9,6 +9,6 @@ repos:
       - id: flake8
         args: ['--max-line-length=140', '--ignore=E203']
   - repo: https://github.com/pre-commit/mirrors-mypy
-    rev: ''  # Use the sha / tag you want to point at
+    rev: 'v0.720'
     hooks:
     -   id: mypy