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

* Fix mypy errors for armstrong_numbers.py

* Fix mypy errors for harmonic_series.py

* Fix mypy errors for average_median.py

Author: imp2002

maths/armstrong_numbers.py

@@ -9,7 +9,7 @@
 On-Line Encyclopedia of Integer Sequences entry: https://oeis.org/A005188
 """
 PASSING = (1, 153, 370, 371, 1634, 24678051, 115132219018763992565095597973971522401)
-FAILING = (-153, -1, 0, 1.2, 200, "A", [], {}, None)
+FAILING: tuple = (-153, -1, 0, 1.2, 200, "A", [], {}, None)
 
 
 def armstrong_number(n: int) -> bool:

maths/average_median.py

@@ -1,14 +1,14 @@
 from typing import Union
 
 
-def median(nums: Union[int, float]) -> Union[int, float]:
+def median(nums: list) -> Union[int, float]:
     """
     Find median of a list of numbers.
     Wiki: https://en.wikipedia.org/wiki/Median
 
     >>> median([0])
     0
-    >>> median([4,1,3,2])
+    >>> median([4, 1, 3, 2])
     2.5
     >>> median([2, 70, 6, 50, 20, 8, 4])
     8

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