Instantiate GridSizer() in a way that works on both Wx 3. and 4.

Looks like GridSizer.SetHGap() and SetVGap() have no effect on Wx 3.
But at least now it doesn't crash on Wx 3.

python/samples/i2cgui.py

@@ -169,7 +169,10 @@ class Frame(wx.Frame):
             r.Bind(wx.EVT_RADIOBUTTON, self.choose_addr)
             return r
         self.heat = {i:addrbutton("%02X" % i) for i in range(0x08, 0x78)}
-        devgrid = wx.GridSizer(8, wx.Size(4, 6))
+        # Be careful here, GridSizer changed between wx 3.x and 4.x
+        devgrid = wx.GridSizer(8)
+        devgrid.SetHGap(4)
+        devgrid.SetVGap(6)
         for i,l in sorted(self.heat.items()):
             devgrid.Add(l)