Hard require Python version 3.7

[srfc]

Hard require Python version 3.7

Due to reliance on datetime.date.fromisoformat which has been added to the standard library with Python 3.7 some functions in cqapi.util do not work with earlier versions of Python 3.

Suggestions

Either hard-require Python version to be >= 3.7.

import sys
if sys.version_info < (2,7):
    sys.exit('Sorry, Python < 2.7 is not supported')

Or parse date strings manually.