test_stats: mock time.time() with a float, to appease windows

For some reason, the appveyor (windows) tests fail on py36 with an OSError
during an internal (twisted.python.log) call to datetime.fromtimestamp().
It's possible that my mock times are too old (I'm using made-up values like
"456"), but maybe it's just that real time.time() returns a float instead of
an integer.

src/wormhole_transit_relay/test/test_stats.py

@@ -15,11 +15,11 @@ class DB(unittest.TestCase):
         d = self.mktemp()
         os.mkdir(d)
         usage_db = os.path.join(d, "usage.sqlite")
-        with mock.patch("time.time", return_value=456):
+        with mock.patch("time.time", return_value=456.0):
             t = Transit(blur_usage=None, log_file=None, usage_db=usage_db)
         db = self.open_db(usage_db)
 
-        with mock.patch("time.time", return_value=457):
+        with mock.patch("time.time", return_value=457.0):
             t.recordUsage(started=123, result="happy", total_bytes=100,
                           total_time=10, waiting_time=2)
         self.assertEqual(db.execute("SELECT * FROM `usage`").fetchall(),
@@ -31,7 +31,7 @@ class DB(unittest.TestCase):
                               incomplete_bytes=0,
                               waiting=0, connected=0))
 
-        with mock.patch("time.time", return_value=458):
+        with mock.patch("time.time", return_value=458.0):
             t.recordUsage(started=150, result="errory", total_bytes=200,
                           total_time=11, waiting_time=3)
         self.assertEqual(db.execute("SELECT * FROM `usage`").fetchall(),
@@ -45,7 +45,7 @@ class DB(unittest.TestCase):
                               incomplete_bytes=0,
                               waiting=0, connected=0))
 
-        with mock.patch("time.time", return_value=459):
+        with mock.patch("time.time", return_value=459.0):
             t.timerUpdateStats()
         self.assertEqual(db.execute("SELECT * FROM `current`").fetchone(),
                          dict(rebooted=456, updated=459,