Why not use strptime?
datetime.datetime.strptime("2007-03-04T21:08:12Z", "%Y-%m-%dT%H:%M:%SZ")
(How do I translate an ISO 8601 datetime string into a Python datetime object? - Stack Overflow)
or even fromisoformat?
https://docs.python.org/3.7/library/datetime.html#datetime.datetime.fromisoformat
Both are untested 
.