Fix naive offset-naive/aware error in asset compilation [CRI-191]

[regisb]

The XBlockPackageStorage used to return offset-naive datetime objects
which were compared to offset-aware objects when we ran static asset
collection:

./manage.py lms collectstatic

Close CRI-191 (which is a Juniper issue, cc @nedbat)

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[nedbat]

@jmbowman @ormsbee Does this look right to you?

[feanil]

Is the timestamp this is going to be connected to always going to be UTC? Should they both be using the timezone.get_current_timezone() function from django.utils.timezone instead? If not, why not?

[regisb]

@feanil It does not matter which timezone is being used to obtain an offset-aware datetime. Using a different timezone would result in a different tz attribute for the datetime, but the absolute datetime would remain identical. This can be verified with the following piece of code:

In [13]: datetime.fromtimestamp(1, timezone.utc)
Out[13]: datetime.datetime(1970, 1, 1, 0, 0, 1, tzinfo=<UTC>)

In [14]: datetime.fromtimestamp(1, timezone.get_fixed_timezone(2))
Out[14]: datetime.datetime(1970, 1, 1, 0, 2, 1, tzinfo=datetime.timezone(datetime.timedelta(0, 120), '+0002'))

However, it does matter that we have an offset-aware datetime in order to be able to perform datetime comparisons while collecting assets. The relevant piece of code is in django/contrib/staticfiles/management/commands/collectstatic.py (line 280):

file_is_unmodified = (
                        target_last_modified.replace(microsecond=0) >=
                        source_last_modified.replace(microsecond=0)
                    )

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