Update infix to postfix (#3817)

* add test to infix_to_postfix_conversion

* fixed pre-commit error

* fixed build error

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

data_structures/stacks/__init__.py

@@ -1,22 +0,0 @@
-class Stack:
-    def __init__(self):
-        self.stack = []
-        self.top = 0
-
-    def is_empty(self):
-        return self.top == 0
-
-    def push(self, item):
-        if self.top < len(self.stack):
-            self.stack[self.top] = item
-        else:
-            self.stack.append(item)
-
-        self.top += 1
-
-    def pop(self):
-        if self.is_empty():
-            return None
-        else:
-            self.top -= 1
-            return self.stack[self.top]