fix: NameError issue with Manager._ref

Since pydantic 2.1, private attributes are not supported anymore.

This fixes this exception:

    NameError: Fields must not use names with leading underscores; e.g., use 'ref' instead of '_ref'.

pydantic_scim2/enterprise_user.py

@@ -10,7 +10,7 @@ class Manager(BaseModel):
         None,
         description="The id of the SCIM resource representingthe User's manager.  REQUIRED.",
     )
-    _ref: Optional[AnyUrl] = Field(
+    ref: Optional[AnyUrl] = Field(
         None,
         alias="$ref",
         description="The URI of the SCIM resource representing the User's manager.  REQUIRED.",

tests/test_responses.py

@@ -22,7 +22,7 @@ def test_user_response(minimal_user_payload):
 
 @pytest.mark.skip
 def test_enterprise_user_response(enterprise_user_payload):
-    from pydantic_scim.enterprise_user import EnterpriseUser
+    from pydantic_scim2.enterprise_user import EnterpriseUser
 
     payload = {
         "totalResults": 1,
@@ -65,7 +65,6 @@ def test_service_provider_configuration_response(
     assert isinstance(obj, ServiceProviderConfiguration)
 
 
-@pytest.mark.skip
 def test_resource_type_response(resource_type_payload):
     payload = {
         "totalResults": 2,