fix: mimic the behavior of stdlib StrEnum.__str__ in backport

Signed-off-by: Federico Bond <federicobond@gmail.com>

Author: federicobond

openfeature/_backports/strenum.py

@@ -11,4 +11,5 @@ class StrEnum(str, Enum):
         Backport StrEnum for Python <3.11
         """
 
-        pass
+        def __str__(self):
+            return str(self.value)

tests/test_exception.py

@@ -0,0 +1,5 @@
+from openfeature.exception import ErrorCode
+
+
+def test_error_code_str():
+    assert str(ErrorCode.GENERAL) == "GENERAL"

tests/test_flag_evaluation.py

@@ -54,3 +54,7 @@ def test_evaluation_details_reason_should_be_a_string_when_set():
 
     # Then
     assert Reason.STATIC == flag_details.reason  # noqa: SIM300
+
+
+def test_reason_str():
+    assert str(Reason.DEFAULT) == "DEFAULT"