Fixes #8754, #8724 Updated postfix_evaluation.py

postfix_evaluation.py now supports Unary operators and floating point numbers.
Also merged evaluate_postfix_notations.py and postfix_evaluation.py into postfix_evaluation.py which fixes #8724. Added a doctest example with unary operator.

data_structures/stacks/postfix_evaluation.py

@@ -88,6 +88,8 @@ def evaluate(post_fix: list[str], verbose: bool = False) -> int | float | str |
     6.6
     >>> evaluate(["2", "-", "3", "+"])
     1
+    >>> evaluate(["-4", "5", "*", "6", "-"])
+    -26
     >>> evaluate([])
     0