Fix missing sys import

[cclauss]

sys.maxsize is accessed on line 31 but sys is not imported.

flake8 testing of https://github.com/donnemartin/interactive-coding-challenges on Python 3.7.0

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./stacks_queues/stack_min/test_stack_min.py:30:39: F821 undefined name 'sys'
        assert_equal(stack.minimum(), sys.maxsize)
                                      ^

[donnemartin]

@cclauss thank you!