Add support for pydantic dataclasses (#406)

tests/inputs/oneof/test_oneof.py

@@ -1,5 +1,6 @@
 import betterproto
 from tests.output_betterproto.oneof import Test
+from tests.output_betterproto_pydantic.oneof import Test as TestPyd
 from tests.util import get_test_case_json_data
 
 
@@ -13,3 +14,8 @@ def test_which_name():
     message = Test()
     message.from_json(get_test_case_json_data("oneof", "oneof_name.json")[0].json)
     assert betterproto.which_one_of(message, "foo") == ("pitier", "Mr. T")
+
+
+def test_which_count_pyd():
+    message = TestPyd(pitier="Mr. T", just_a_regular_field=2, bar_name="a_bar")
+    assert betterproto.which_one_of(message, "foo") == ("pitier", "Mr. T")