Migrate to StrEnum to fix ruff UP042

Replace `class Foo(str, Enum)` with `class Foo(StrEnum)` for MeterType
and SensorState, fixing the new UP042 lint rule in ruff 0.15.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: danieldotnl

custom_components/measureit/const.py

@@ -1,6 +1,6 @@
 """Constants for MeasureIt."""
 
-from enum import Enum
+from enum import StrEnum
 from logging import Logger, getLogger
 
 LOGGER: Logger = getLogger(__package__)
@@ -58,15 +58,15 @@
 }
 
 
-class MeterType(str, Enum):
+class MeterType(StrEnum):
     """Enum with possible meter states."""
 
     TIME = "time"
     SOURCE = "source"
     COUNTER = "counter"
 
 
-class SensorState(str, Enum):
+class SensorState(StrEnum):
     """Enum with possible meter states."""
 
     INITIALIZING_SOURCE = "initializing source value"