Add mypy to ci and precommit (#5969)

* Add mypy to ci and precommit

* Run precommit mypy in venv

* Fix issues raised in latest version of mypy

script/run-in-env.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env sh
+set -eu
+
+# Used in venv activate script.
+# Would be an error if undefined.
+OSTYPE="${OSTYPE-}"
+
+# Activate pyenv and virtualenv if present, then run the specified command
+
+# pyenv, pyenv-virtualenv
+if [ -s .python-version ]; then
+    PYENV_VERSION=$(head -n 1 .python-version)
+    export PYENV_VERSION
+fi
+
+if [ -n "${VIRTUAL_ENV-}" ] && [ -f "${VIRTUAL_ENV}/bin/activate" ]; then
+  . "${VIRTUAL_ENV}/bin/activate"
+else
+  # other common virtualenvs
+  my_path=$(git rev-parse --show-toplevel)
+
+  for venv in venv .venv .; do
+    if [ -f "${my_path}/${venv}/bin/activate" ]; then
+      . "${my_path}/${venv}/bin/activate"
+      break
+    fi
+  done
+fi
+
+exec "$@"