Support spaces separaters in ISO-8601 datetimes between times and timezone strings

[Ben Wilber]

GitHub Pull Request: ​https://github.com/django/django/pull/14368

According to ISO-8601, there can be any number of whitespace characters between the time strings and timezone strings.

Unfortunately the spec isn't public, but here's the link anyway ​https://www.iso.org/iso-8601-date-and-time-format.html.

Examples:

This is a valid ISO-8601 datetime string:

2012-04-23T10:20:30.400-02

django.utils.dateparse.parse_datetime parses this correctly.

This is also a valid ISO-8601 datetime string:

2012-04-23T10:20:30.400 -02

django.utils.dateparse.parse_datetime does not parse this correctly and returns None,

However, python-dateutil parses it correctly. The difference is that Django uses a (brittle) regex to parse ISO-8601 datetime strings, and python-dateutil does not.

​https://github.com/django/django/blob/main/django/utils/dateparse.py#L22
​https://github.com/dateutil/dateutil/blob/master/dateutil/parser/isoparser.py

I recommend that Django:

1) Depend on python-dateutil for datetime string parsing

OR

2) Inline python-dateutils' parsing functions

As far as I know there is no regex that can parse the full spec of ISO-8601 datetime strings.

In the meantime, this is a patch to support (valid) whitespace characters between the seconds/millseconds part and the timezone string.