BUG: Fix declarative show() (Fixes #1207)

The goal of the original implementation was to avoid triggering
matplotlib backend detection code on import by avoiding importing the
pyplot interface. This seems no longer necessary, and more importantly
it doesn't work just to use figure.show(), so just use plt.show()

src/metpy/plots/declarative.py

@@ -568,7 +568,7 @@ def save(self, *args, **kwargs):
     def show(self):
         """Show the constructed graphic on the screen."""
         self.draw()
-        self.figure.show()
+        plt.show()
 
 
 @exporter.export