Update pop function (#5544)

* Updated Pop function

Added underflow condition

* Update Pop Function

Added condition to check underflow of stack

* Update stack.py

* if not self.stack: raise StackUnderflowError

* Add doctests

* StackUnderflowError

* ..., not ....

* Update stack.py

Co-authored-by: Christian Clauss <cclauss@me.com>