Use @keyword_only for graphics_array.save() and .show().

This is based on ticket #15515 comment 16, which was partly undone
by commit 7747261 from #17234. Since neither ticket discussed an
official deprecation (the former left things undeprecated, the latter
simply removed positional functionality with no contingency), the
deprecation warnings will refer to 16607 where this can be discussed.

src/sage/plot/graphics.py

@@ -3400,7 +3400,8 @@ def append(self, g):
  # Not clear if there is a way to do this
  raise NotImplementedError('Appending to a graphics array is not yet implemented')
 
- def save(self, filename=None, dpi=DEFAULT_DPI, figsize=None, axes=None,
+ @keyword_only(positional=4, deprecation=16607, extra="axes")
+ def save(self, filename=None, dpi=DEFAULT_DPI, figsize=None,
  **kwds):
  r"""
  Save the graphics array.
@@ -3467,9 +3468,9 @@ def save(self, filename=None, dpi=DEFAULT_DPI, figsize=None, axes=None,
  for i,g in zip(range(1, dims+1), glist):
  subplot = figure.add_subplot(rows, cols, i)
  g.matplotlib(filename, figure=figure, sub=subplot,
- verify=do_verify, axes = axes, **kwds)
+ verify=do_verify, **kwds)
  g.save(filename, dpi=dpi, figure=figure, sub=subplot,
- verify=do_verify, axes=axes, **kwds)
+ verify=do_verify, **kwds)
 
  def save_image(self, filename=None, *args, **kwds):
  r"""
@@ -3493,6 +3494,7 @@ def save_image(self, filename=None, *args, **kwds):
  """
  self.save(filename, *args, **kwds)
 
+ @keyword_only(deprecation=16607, extra="filename,dpi,figsize,axes")
  def show(self, **kwds):
  r"""
  Show this graphics array immediately.