code: def are_requirements_changed(config):
      
      deps = (dep.name for dep in config.deps)
  
      def build_fpath_for_previous_version(fname):
          tox_dir = config.config.toxworkdir.strpath
          envdirkey = _str_to_sha1hex(str(config.envdir))
          fname = '{0}.{1}.previous'.format(fname.replace('/', '-'), envdirkey)
          return os.path.join(tox_dir, fname)
  
      requirement_files = map(parse_requirements_fname, deps)
      return any([
          is_changed(reqfile, build_fpath_for_previous_version(reqfile))
          for reqfile in requirement_files
          if reqfile and os.path.isfile(reqfile)])
docs: Check if any of the requirement files used by testenv is updated.
  
      :param tox.config.TestenvConfig config: Configuration object to observe.
      :rtype: bool