Add basic mypy linting

Based on pytest setup, though more lax. Currently only the tests are
annotated so only helpful for them.

Fix #721.

.pre-commit-config.yaml

@@ -28,3 +28,12 @@ repos:
         files: ^(CHANGELOG.rst|HOWTORELEASE.rst|README.rst|changelog/.*)$
         language: python
         additional_dependencies: [pygments, restructuredtext_lint]
+-   repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.910-1
+    hooks:
+    -   id: mypy
+        files: ^(src/|testing/)
+        args: []
+        additional_dependencies:
+          - pytest>=6.2.0
+          - py>=1.10.0