Fixed #21487 -- Session cannot store tzinfo instances anymore.

Add the following middleware to :setting:`MIDDLEWARE_CLASSES`::

    import pytz

    from django.utils import timezone

    class TimezoneMiddleware(object):

        def process_request(self, request):

            tz = request.session.get('django_timezone')

            if tz:

                timezone.activate(tz)

            tzname = request.session.get('django_timezone')

            if tzname:

                timezone.activate(pytz.timezone(tzname))

            else:

                timezone.deactivate()

Create a view that can set the current timezone::

    import pytz

    from django.shortcuts import redirect, render

    def set_timezone(request):

        if request.method == 'POST':

            request.session['django_timezone'] = pytz.timezone(request.POST['timezone'])

            request.session['django_timezone'] = request.POST['timezone']

            return redirect('/')

        else:

            return render(request, 'template.html', {'timezones': pytz.common_timezones})