Remove dateutil as dependency

[fbernhart]

python-dateutil is one of the few dependencies.

It's only used twice in the pypi.py file to create datetime objects from strings. E.g.:

version_date = dateutil.parser.parse(version_date)

But the same can as well easily be achieved with the standard library datetime module:

version_date = datetime.strptime(version_date, "%Y-%m-%dT%H:%M:%S.%fZ")

So this PR aims at removing python-dateutil as dependency and using datetime instead.

[miketheman]

I checked this out and tested this today - it works as described, and reduces reliance on one external dependency.