Mypy (#217)

* Mypy: Initial support

 * Add to pre-commit (currently installs own deps, could maybe changed
   to poetry venv in the future to reuse environment and don't need
   duplicated types deps).
 * Add type hints.

* Mypy: Add missing annotations

src/akkudoktoreos/utils/utils.py

@@ -1,12 +1,13 @@
 import datetime
 import json
 import zoneinfo
+from typing import Any
 
 import numpy as np
 
 
 # currently unused
-def ist_dst_wechsel(tag: datetime.datetime, timezone="Europe/Berlin") -> bool:
+def ist_dst_wechsel(tag: datetime.datetime, timezone: str = "Europe/Berlin") -> bool:
     """Checks if Daylight Saving Time (DST) starts or ends on a given day."""
     tz = zoneinfo.ZoneInfo(timezone)
     # Get the current day and the next day
@@ -20,15 +21,25 @@ def ist_dst_wechsel(tag: datetime.datetime, timezone="Europe/Berlin") -> bool:
 
 
 class NumpyEncoder(json.JSONEncoder):
-    def default(self, obj):
+    @classmethod
+    def convert_numpy(cls, obj: Any) -> tuple[Any, bool]:
         if isinstance(obj, np.ndarray):
-            return obj.tolist()  # Convert NumPy arrays to lists
+            # Convert NumPy arrays to lists
+            return [
+                None if isinstance(x, (int, float)) and np.isnan(x) else x for x in obj.tolist()
+            ], True
         if isinstance(obj, np.generic):
-            return obj.item()  # Convert NumPy scalars to native Python types
+            return obj.item(), True  # Convert NumPy scalars to native Python types
+        return obj, False
+
+    def default(self, obj: Any) -> Any:
+        obj, converted = NumpyEncoder.convert_numpy(obj)
+        if converted:
+            return obj
         return super(NumpyEncoder, self).default(obj)
 
     @staticmethod
-    def dumps(data):
+    def dumps(data: Any) -> str:
         """Static method to serialize a Python object into a JSON string using NumpyEncoder.
 
         Args: