refactor(report): Clean _extract_timestamp method code

Author: gfieni

src/powerapi/report/report.py

@@ -109,15 +109,23 @@ def to_json(report: Report) -> Dict:
 
     @staticmethod
     def _extract_timestamp(ts):
+        # Unix timestamp format (in milliseconds)
+        if isinstance(ts, int):
+            return datetime.fromtimestamp(ts / 1000)
+
+        # datetime object
+        if isinstance(ts, datetime):
+            return ts
+
         if isinstance(ts, str):
             try:
+                # ISO 8601 date format
                 return datetime.strptime(ts, "%Y-%m-%dT%H:%M:%S.%f")
             except ValueError:
+                # Unix timestamp format (in milliseconds)
                 return datetime.fromtimestamp(int(ts) / 1000)
-        if isinstance(ts, datetime):
-            return ts
 
-        raise ValueError('timestamp must be a datetime.datetime or a string')
+        raise ValueError('Invalid timestamp format')
 
     @staticmethod
     def create_empty_report():