Corrected calculation of UserSocialAuthMixin.expiration_datetime

[barracel]

Corrected calculation of "now timestamp" in utc.

datetime.now() returns a datetime in local time zone so by doing now.replace(tzinfo=utc) you are adding/removing time (depending on your configured local timezone)
I changed it to use now_timestamp because it is calculated with time.mktime and it takes into account time zones.

If you were if the side of the word where "UTC offset > than provider session time" the function UserSocialAuthMixin.expiration_datetime returned a negative time that forced the expiration of the django session.

[omab]

@barracel, what do you think about the code used on python-social-auth here https://github.com/omab/python-social-auth/blob/master/social/storage/base.py#L55? I was going to port it to django-social-auth shortly.

[barracel]

That's even better :)

[omab]

I've ported it, thanks!