Implement __str__ for enum props to fix conversion to query string

Author: packyg

openapi_python_client/templates/int_enum.pyi

@@ -4,3 +4,6 @@ class {{ enum.reference.class_name }}(IntEnum):
     {% for key, value in enum.values.items() %}
     {{ key }} = {{ value }}
     {% endfor %}
+
+    def __str__(self) -> str:
+        return str(self.value)

openapi_python_client/templates/str_enum.pyi

@@ -4,3 +4,6 @@ class {{ enum.reference.class_name }}(str, Enum):
     {% for key, value in enum.values.items() %}
     {{ key }} = "{{ value }}"
     {% endfor %}
+
+    def __str__(self) -> str:
+        return str(self.value)