g.gui.iclass: replace dist_point_to_segment()

from matplotlib, which was disabled for matplotlib 3.1.0+.

Fixes: #461

gui/wxpython/iscatt/plots.py

@@ -35,7 +35,6 @@
         FigureCanvasWxAgg as FigCanvas
     from matplotlib.lines import Line2D
     from matplotlib.artist import Artist
-    from matplotlib.mlab import dist_point_to_segment
     from matplotlib.patches import Polygon, Ellipse, Rectangle
     import matplotlib.image as mi
     import matplotlib.colors as mcolors
@@ -838,6 +837,13 @@ def _deleteVertex(self, event):
 
         self.Redraw()
 
+    def _dist_point_to_segment(p, s0, s1):
+        p = np.asarray(p, float)
+        s0 = np.asarray(s0, float)
+        s1 = np.asarray(s1, float)
+
+        return np.cross(s1-s0, p-s0)/np.linalg.norm(s1-s0)
+
     def _addVertexOnBoundary(self, event):
         if self.empty_pol:
             return
@@ -847,7 +853,7 @@ def _addVertexOnBoundary(self, event):
         for i in range(len(xys) - 1):
             s0 = xys[i]
             s1 = xys[i + 1]
-            d = dist_point_to_segment(p, s0, s1)
+            d = self._dist_point_to_segment(p, s0, s1)
 
             if d <= self._getSnapTresh():
                 self.pol.xy = np.array(