maint(dev): pin pydantic version

v2 of pydantic introduces breaking changes, specifically Field()
now returns a FieldInfo with the `annotation` field set to NonType.

Author: gnikit

setup.cfg

@@ -62,7 +62,7 @@ dev =
     black
     isort
     pre-commit
-    pydantic
+    pydantic==1.9.1
 docs =
     sphinx >= 4.0.0
     sphinx-argparse