title: Change units of matplotlib subplot (RA hh:mm to degrees)
body: <p>I am trying to plot an image with axes in RA and DEC. I cannot figure out how to change units from h:m:s to deg for my RA axis. I tried the following 2 methods and my output axes were the same (ticks: 5h 42m 15s, 00, 41m 45s, ...)</p>
  <pre><code>filename = get_pkg_data_filename('flame.fits')
  hdu = fits.open(filename)[0]
  wcs = WCS(hdu.header)
  
  fig = plt.figure()
  fig.add_subplot(111, projection=wcs)
  plt.imshow(hdu.data, origin='lower', cmap=plt.cm.viridis)
  plt.xlabel('RA')
  plt.ylabel('Dec')
  plt.show()
  </code></pre>
  <p>METHOD 2:</p>
  <pre><code>flame = fits.getdata('flame.fits')
  hdr   = fits.getheader('flame.fits')
  wcs   = WCS(hdr) 
  
  ax = plt.subplot(projection=wcs)
  plt.imshow(flame, origin='lower')
  plt.xlabel('RA')
  plt.ylabel('Dec')
  plt.show()
  </code></pre>
  <p>How do I change the X-axis (RA) to degrees?
  Thanks!</p>
  <p>P.S.:
  This</p>
  <pre><code>ax = fig.gca()
  ra = ax.coords[0]
  dec = ax.coords[1]
  ra.set_format_unit('ra')
  dec.set_format_unit('dec')
  </code></pre>
  <p>result in <strong>AttributeError: 'AxesSubplot' object has no attribute 'coords'</strong></p>