code: def _find_rtd_version():
      
      vstr = 'latest'
      try:
          import ginga
          from bs4 import BeautifulSoup
      except ImportError:
          return vstr
  
      # No active doc build before this release, just use latest.
      if not minversion(ginga, '2.6.0'):
          return vstr
  
      # Get RTD download listing.
      url = 'https://readthedocs.org/projects/ginga/downloads/'
      with urllib.request.urlopen(url) as r:
          soup = BeautifulSoup(r, 'html.parser')
  
      # Compile a list of available HTML doc versions for download.
      all_rtd_vernums = []
      for link in soup.find_all('a'):
          href = link.get('href')
          if 'htmlzip' not in href:
              continue
          s = href.split('/')[-2]
          if s.startswith('v'):  # Ignore latest and stable
              all_rtd_vernums.append(s)
      all_rtd_vernums.sort(reverse=True)
  
      # Find closest match.
      ginga_ver = ginga.__version__
      for rtd_ver in all_rtd_vernums:
          if ginga_ver > rtd_ver[1:]:  # Ignore "v" in comparison
              break
          else:
              vstr = rtd_ver
  
      return vstr
docs: Find closest RTD doc version.