MNT: Fix test to actually check numerics

No clue why it was ever deemed a good idea to use string representation
of a value to check this (maybe no numpy dependence at the time?), but
it's breaking on Python 3.7. Move to a proper test with
assert_almost_equal.

siphon/tests/test_ncss.py

@@ -184,7 +184,7 @@ def test_vertical_level(self):
         self.nq.accept('csv').vertical_level(50000)
         csv_data = self.ncss.get_data(self.nq)
 
-        assert str(csv_data['Temperature_isobaric'])[:6] == '263.39'
+        np.testing.assert_almost_equal(csv_data['Temperature_isobaric'], np.array([263.40]), 2)
 
     @recorder.use_cassette('ncss_gfs_csv_point')
     def test_raw_csv(self):