[mypy] Fix type annotations for stack.py

[archaengel]

$ git checkout mypy-fix-stacks-stack
Switched to branch 'mypy-fix-stacks-stack'

$ mypy --ignore-missing-imports data_structures/stacks/stack.py --strict
Success: no issues found in 1 source file

$ mypy --ignore-missing-imports data_structures/stacks --strict > after.txt

$ git checkout master
Switched to branch 'master'

$ mypy --ignore-missing-imports data_structures/stacks --strict > before.txt

$ diff before.txt after.txt
39,49d38
< data_structures/stacks/stack.py:31: error: Function is missing a type annotation
< data_structures/stacks/stack.py:37: error: Function is missing a return type annotation
< data_structures/stacks/stack.py:50: error: Function is missing a return type annotation
< data_structures/stacks/stack.py:74: error: Function is missing a type annotation for one or more arguments
< data_structures/stacks/stack.py:90: error: Call to untyped function "pop" in typed context
< data_structures/stacks/stack.py:96: error: Call to untyped function "peek" in typed context
< data_structures/stacks/stack.py:103: error: Call to untyped function "push" in typed context
< data_structures/stacks/stack.py:109: error: Call to untyped function "pop" in typed context
< data_structures/stacks/stack.py:110: error: Call to untyped function "peek" in typed context
< data_structures/stacks/stack.py:112: error: Call to untyped function "push" in typed context
< data_structures/stacks/stack.py:116: error: Call to untyped function "push" in typed context
52,62d40
< data_structures/stacks/dijkstras_two_stack_algorithm.py:60: error: Call to untyped function "push" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:63: error: Call to untyped function "push" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:66: error: Call to untyped function "peek" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:67: error: Call to untyped function "pop" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:68: error: Call to untyped function "peek" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:69: error: Call to untyped function "pop" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:70: error: Call to untyped function "peek" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:71: error: Call to untyped function "pop" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:74: error: Call to untyped function "push" in typed context
< data_structures/stacks/dijkstras_two_stack_algorithm.py:77: error: Returning Any from function declared to return "int"
< data_structures/stacks/dijkstras_two_stack_algorithm.py:77: error: Call to untyped function "peek" in typed context
75,85c53
< data_structures/stacks/balanced_parentheses.py:21: error: Call to untyped function "push" in typed context
< data_structures/stacks/balanced_parentheses.py:23: error: Call to untyped function "pop" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:47: error: Call to untyped function "push" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:49: error: Call to untyped function "peek" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:50: error: Call to untyped function "pop" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:51: error: Call to untyped function "pop" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:53: error: Call to untyped function "peek" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:54: error: Call to untyped function "pop" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:55: error: Call to untyped function "push" in typed context
< data_structures/stacks/infix_to_postfix_conversion.py:57: error: Call to untyped function "pop" in typed context
< Found 82 errors in 12 files (checked 13 source files)
---
> Found 50 errors in 8 files (checked 13 source files)

Related to #4052

Describe your change:

Add generic type annotations to functions in stack.py. Add type annotations to variables of Stack class.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[cclauss]

I thought that the reason that we used Stack(Generic[T]) instead of Stack(Any) was so that we did not need to declare the type of each instance at creation time.