use keyword-only params in sort_index

in methods where inplace is allowed, non-keyword args will become deprecated soon:
pandas-dev/pandas#41506

This does not change any current behavior, but removes a deprecation warning when using the latest pandas version

Author: maurerle

entsoe/entsoe.py

@@ -1911,7 +1911,7 @@ def query_generation_per_plant(
         df = df.tz_convert(area.tz)
         # Truncation will fail if data is not sorted along the index in rare
         # cases. Ensure the dataframe is sorted:
-        df = df.sort_index(0)
+        df = df.sort_index(axis=0)
         df = df.truncate(before=start, after=end)
         return df