Add missing enum imports

Created by: cclauss

Resolve the undefined name Enum in two files by importing it from enum. Also fix a Syntax Error.

flake8 testing of https://github.com/donnemartin/system-design-primer on Python 3.6

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

./solutions/object_oriented_design/online_chat/online_chat.py:98:21: F821 undefined name 'Enum'
class RequestStatus(Enum):
                    ^
./solutions/system_design/mint/mint_snippets.py:3:25: F821 undefined name 'Enum'
class DefaultCategories(Enum):
                        ^
2    F821 undefined name 'Enum'
2