switch to pydantic

Signed-off-by: Jess Frazelle <github@jessfraz.com>

kittycad/models/country_code.py

@@ -1,3 +1,17 @@
+from typing import Any
+
+from pydantic import GetCoreSchemaHandler
+from pydantic_core import CoreSchema, core_schema
+
+
 class CountryCode(str):
+    """An ISO-3166 alpha-2 country code. Always uppercase."""
+
     def __str__(self) -> str:
         return self
+
+    @classmethod
+    def __get_pydantic_core_schema__(
+        cls, source_type: Any, handler: GetCoreSchemaHandler
+    ) -> CoreSchema:
+        return core_schema.no_info_after_validator_function(cls, handler(str))