Use better date/time validation

[sajith]

We're using regular expressions to validate date and time strings:

datamodel/sdxdatamodel/validation/connectionvalidator.py

Lines 12 to 14 in 3a83c58

	ISO_FORMAT = r'\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}[-+]\d{2}:\d{2}'
	ISO_TIME_FORMAT = r"(^(?:[1-9]\d{3}-(?:(?:0[1-9]|1[0-2])-(?:0[1-9]|1\d|2[0-8])|(?:0[13-9]|1[0-2])-(?:29|30)|(?:0[13578]|1[02])-31)|(?:[1-9]\d(?:0[48]|[2468][048]|[13579][26])|(?:[2468][048]|[13579][26])00)-02-29)T(?:[01]\d|2[0-3]):[0-5]\d:[0-5]\d(?:Z|[+-][01]\d:[0-5]\d)$)"

datamodel/sdxdatamodel/validation/connectionvalidator.py

Line 107 in 3a83c58

if not match(ISO_TIME_FORMAT, time):

datamodel/sdxdatamodel/validation/topologyvalidator.py

Line 19 in 3a83c58

ISO_FORMAT = r'\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}[-+]\d{2}:\d{2}'

datamodel/sdxdatamodel/validation/topologyvalidator.py

Lines 111 to 123 in 3a83c58

	elif not match(ISO_FORMAT, version):
	errors.append(
	'{} Version must be datetime ISO format'.format(
	topology.id,
	)
	)
	if not match(ISO_FORMAT, time_stamp):
	errors.append(
	'{} time_stamp needs to be in full ISO format'.format(
	time_stamp,
	)
	)

But this is harder to understand, and they do not really check for invalid data (such as February 31). We probably should be using a real date/time parser. From this StackOverflow question, one way seems to be using datetime library:

from datetime import datetime

def datetime_valid(dt_str):
    try:
        datetime.fromisoformat(dt_str.replace('Z', '+00:00'))
    except:
        return False
    return True

However datetime's own documentation suggests using isoparse from third-party dateutil library.