fix deprecated features in matplotlib >=3.2 (#110)

gym_minigrid/window.py

@@ -26,8 +26,10 @@ class Window:
         self.fig.canvas.set_window_title(title)
 
         # Turn off x/y axis numbering/ticks
-        self.ax.set_xticks([], [])
-        self.ax.set_yticks([], [])
+        self.ax.xaxis.set_ticks_position('none')
+        self.ax.yaxis.set_ticks_position('none')
+        _ = self.ax.set_xticklabels([])
+        _ = self.ax.set_yticklabels([])
 
         # Flag indicating the window was closed
         self.closed = False