MNT: Fix some of the polygon tests

These were relying on the order of polygons, which doesn't seem to be
guaranteed by the wrapping process. It was essentially relying upon the
order of popitem() from a dict, which changed with Python 3.6. This
changes to try to identify the appropriate polygons before embarking on
testing them.

lib/cartopy/tests/test_polygon.py

@@ -270,10 +270,13 @@ def test_plate_carree_partial_wrap(self):
         # Check the structure
         assert len(multi_polygon) == 2
         # Check the rough shape
-        polygon = multi_polygon[0]
-        self._assert_bounds(polygon.bounds, 170, 0, 180, 10)
-        polygon = multi_polygon[1]
-        self._assert_bounds(polygon.bounds, -180, 0, -170, 10)
+        poly1, poly2 = multi_polygon
+        # The order of these polygons is not guaranteed, so figure out
+        # which is appropriate
+        if 170.0 not in poly1.bounds:
+            poly1, poly2 = poly2, poly1
+        self._assert_bounds(poly1.bounds, 170, 0, 180, 10)
+        self._assert_bounds(poly2.bounds, -180, 0, -170, 10)
 
     def test_plate_carree_wrap(self):
         proj = ccrs.PlateCarree()
@@ -312,14 +315,19 @@ def test_wrapped_poly_simple_hole(self):
         multi_polygon = proj.project_geometry(poly)
         # Check the structure
         assert len(multi_polygon) == 2
-        assert len(multi_polygon[0].interiors) == 1
-        assert len(multi_polygon[1].interiors) == 0
+
+        poly1, poly2 = multi_polygon
+        # The order of these polygons is not guaranteed, so figure out
+        # which is appropriate
+        if not len(poly1.interiors) == 1:
+            poly1, poly2 = poly2, poly1
+
+        assert len(poly1.interiors) == 1
+        assert len(poly2.interiors) == 0
         # Check the rough shape
-        polygon = multi_polygon[0]
-        self._assert_bounds(polygon.bounds, 110, -47, 180, 47)
-        self._assert_bounds(polygon.interiors[0].bounds, 130, -21, 170, 21)
-        polygon = multi_polygon[1]
-        self._assert_bounds(polygon.bounds, -180, -43, -170, 43)
+        self._assert_bounds(poly1.bounds, 110, -47, 180, 47)
+        self._assert_bounds(poly1.interiors[0].bounds, 130, -21, 170, 21)
+        self._assert_bounds(poly2.bounds, -180, -43, -170, 43)
 
     def test_wrapped_poly_wrapped_hole(self):
         proj = ccrs.PlateCarree(-180)