Run mypy through a pyenv/virtualenv enabler wrapper script (#30922)

script/run-in-env.sh

@@ -0,0 +1,18 @@
+#!/bin/sh -eu
+
+# 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
+
+# other common virtualenvs
+for venv in venv .venv .; do
+    if [ -f $venv/bin/activate ]; then
+        . $venv/bin/activate
+    fi
+done
+
+exec "$@"