fix: energy charts bidding zone in request (#948)

Ensure that the bidding zone in the request is correctly set to a
string value (not an enum).

This seems to be also an issue with python version < 3.11. Add safeguards
to only use python >= 3.11. Still keep a regression test for the enum
conversion to string.

Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>

src/akkudoktoreos/core/ems.py

@@ -1,7 +1,7 @@
 import traceback
 from asyncio import Lock, get_running_loop
 from concurrent.futures import ThreadPoolExecutor
-from enum import Enum
+from enum import StrEnum
 from functools import partial
 from typing import ClassVar, Optional
 
@@ -30,7 +30,7 @@ from akkudoktoreos.utils.datetimeutil import DateTime, to_datetime
 executor = ThreadPoolExecutor(max_workers=1)
 
 
-class EnergyManagementStage(Enum):
+class EnergyManagementStage(StrEnum):
     """Enumeration of the main stages in the energy management lifecycle."""
 
     IDLE = "IDLE"
@@ -39,10 +39,6 @@ class EnergyManagementStage(Enum):
     OPTIMIZATION = "OPTIMIZATION"
     CONTROL_DISPATCH = "CONTROL_DISPATCH"
 
-    def __str__(self) -> str:
-        """Return the string representation of the stage."""
-        return self.value
-
 
 async def ems_manage_energy() -> None:
     """Repeating task for managing energy.
@@ -193,7 +189,7 @@ class EnergyManagement(
             None
         """
         # Ensure there is only one optimization/ energy management run at a time
-        if not EnergyManagementMode.is_valid(mode):
+        if not mode in EnergyManagementMode._value2member_map_:
             raise ValueError(f"Unknown energy management mode {mode}.")
         if mode == EnergyManagementMode.DISABLED:
             return