[mypy] Fix type annotations for maths (#4617)

* Fix mypy errors for armstrong_numbers.py

* Fix mypy errors for harmonic_series.py

* Fix mypy errors for average_median.py

maths/series/harmonic_series.py

@@ -33,8 +33,8 @@ def harmonic_series(n_term: str) -> list:
     ['1']
     """
     if n_term == "":
-        return n_term
-    series = []
+        return []
+    series: list = []
     for temp in range(int(n_term)):
         series.append(f"1/{temp + 1}" if series else "1")
     return series