Handle string timestamps with timezone in format ±hh:mm

[danypype]

Updated the humanizeTimestamp function to properly handle string timestamps with the timezone in format ±hh:mm which is also ISO 8601-compliant

Reason for the change:

The python SDK returns activity.time as a datetime.datetime object in feed.get, when these objects are sent as part of a JSON API payload they get converted to an ISO string with timezone in format ±hh:mm

>>> datetime.datetime(2021, 12, 6, 22, 35, 43, 345000, tzinfo=pytz.UTC).isoformat()
'2021-12-06T22:35:43.345000+00:00'

[arnautov-anton]

Hey, @danypype! Thank you for your contribution, this seems like a very reasonable feature to add so it's a :heavy_check_mark: from me. :+1:

[danypype]

😄