drop undocumented dependency on dateutil (#4266)

It turns out we were relying on dateutil being pulled in transitively by
pysaml2. There's no need for that bloat.

changelog.d/4266.misc

@@ -0,0 +1 @@
+drop undocumented dependency on dateutil

synapse/storage/__init__.py

@@ -14,12 +14,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import datetime
+import calendar
 import logging
 import time
 
-from dateutil import tz
-
 from synapse.api.constants import PresenceState
 from synapse.storage.devices import DeviceStore
 from synapse.storage.user_erasure_store import UserErasureStore
@@ -357,10 +355,11 @@ class DataStore(RoomMemberStore, RoomStore,
         """
         Returns millisecond unixtime for start of UTC day.
         """
-        now = datetime.datetime.utcnow()
-        today_start = datetime.datetime(now.year, now.month,
-                                        now.day, tzinfo=tz.tzutc())
-        return int(time.mktime(today_start.timetuple())) * 1000
+        now = time.gmtime()
+        today_start = calendar.timegm((
+            now.tm_year, now.tm_mon, now.tm_mday, 0, 0, 0,
+        ))
+        return today_start * 1000
 
     def generate_user_daily_visits(self):
         """