docs: def get_es_requirements(es_version):
      '''Get the requirements string for elasticsearch-py library
  
      Returns a suitable requirements string for the elsaticsearch-py library
      according to the elasticsearch version to be supported (es_version)'''
  
      # accepts version range in the form `2.x`
      es_version = es_version.replace('x', '0')
      es_version = map(int, es_version.split('.'))
      if es_version >= [6]:
          return ">=6.0.0, <7.0.0"
      elif es_version >= [5]:
          return ">=5.0.0, <6.0.0"
      elif es_version >= [2]:
          return ">=2.0.0, <3.0.0"
      elif es_version >= [1]:
          return ">=1.0.0, <2.0.0"
      else:
          return "<1.0.0"