Remove blanket mypy exclusion from pre-commit config

Author: correctmost

.pre-commit-config.yaml

@@ -65,7 +65,7 @@ repos:
         args: []
         require_serial: true
         additional_dependencies: ["types-typed-ast"]
-        exclude: tests/testdata| # exclude everything, we're not ready
+        exclude: tests/testdata
   - repo: https://github.com/pre-commit/mirrors-prettier
     rev: v4.0.0-alpha.8
     hooks: