Fix "method_complexity" issue in hl7/xml/datatypes.py

[weynelucas]

Function parse_datetime has a Cognitive Complexity of 17 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/weynelucas/python-xml-hl7/hl7/xml/datatypes.py#issue_5e30622640360e0001000019