getClusters returns empty list when specified area crosses 180°E/W

[LincolnPuzey]

When the specified area to getClusters crosses 180°E/W, no clusters are returned.
(e.g. left side = 170°E and right side = 170°W)

I have adapted the example in the README to show the problem.

import numpy
import pysupercluster

# some points near 180°E/W 1°N
points = numpy.array([
    (179.9900, 1.0001),
    (-179.8800, 1.0005),
    (179.9950, 1.0009),
    (-179.9910, 1.0003),
])
index = pysupercluster.SuperCluster(
    points,
    min_zoom=0,
    max_zoom=16,
    radius=40,
    extent=512,
)
clusters = index.getClusters(
    # area that covers all the points and crosses 180°E/W
    top_left=(170, 10),
    bottom_right=(-170, -10),
    zoom=4,
)
print(clusters)  # [] returned when there should be clusters