Use pytz again for tests

This is because datetime.timezone is Python 3 only.  The only
alternative would be to hand-spin a datetime.tzinfo subclass, an
overkill.

This reverts commit 0e6cab3576.

tests/test_helpers.py

@@ -23,6 +23,7 @@ from werkzeug.http import parse_cache_control_header, parse_options_header
 from werkzeug.http import http_date
 from flask._compat import StringIO, text_type
 from flask.helpers import get_debug_flag, make_response
+from pytz import timezone
 
 
 def has_encoding(name):
@@ -177,14 +178,14 @@ class TestJSON(object):
             assert rv.mimetype == 'application/json'
             assert flask.json.loads(rv.data)['x'] == http_date(d.timetuple())
 
-    @pytest.mark.parametrize('tz', (('UTC', 0), ('PST', -8), ('KST', 9)))
-    def test_jsonify_aware_datetimes(self, tz):
+    @pytest.mark.parametrize('tzname', ('UTC', 'PST8PDT', 'Asia/Seoul'))
+    def test_jsonify_aware_datetimes(self, tzname):
         """Test if aware datetime.datetime objects are converted into GMT."""
-        tzinfo = datetime.timezone(datetime.timedelta(hours=tz[1]), name=tz[0])
-        dt = datetime.datetime(2017, 1, 1, 12, 34, 56, tzinfo=tzinfo)
-        gmt = datetime.timezone(datetime.timedelta(), name='GMT')
-        expected = dt.astimezone(gmt).strftime('"%a, %d %b %Y %H:%M:%S %Z"')
-        assert flask.json.JSONEncoder().encode(dt) == expected
+        dt_naive = datetime.datetime(2017, 1, 1, 12, 34, 56)
+        dt_aware = timezone(tzname).localize(dt_naive)
+        dt_as_gmt = dt_aware.astimezone(timezone('GMT'))
+        expected = dt_as_gmt.strftime('"%a, %d %b %Y %H:%M:%S %Z"')
+        assert flask.json.JSONEncoder().encode(dt_aware) == expected
 
     def test_jsonify_uuid_types(self, app, client):
         """Test jsonify with uuid.UUID types"""

tox.ini

@@ -15,6 +15,7 @@ deps =
     coverage
     greenlet
     blinker
+    pytz
 
     lowest: Werkzeug==0.9
     lowest: Jinja2==2.4