code: def get_default_envlist(version):
      
      if version in ['pypy', 'pypy3']:
          return version
  
      # Assume single digit major and minor versions
      match = re.match(r'^(\d)\.(\d)(?:\.\d+)?$', version or '')
      if match:
          major, minor = match.groups()
          return 'py{major}{minor}'.format(major=major, minor=minor)
  
      return guess_python_env()
docs: Parse a default tox env based on the version.
  
      The version comes from the ``TRAVIS_PYTHON_VERSION`` environment
      variable. If that isn't set or is invalid, then use
      sys.version_info to come up with a reasonable default.