<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_default_envlist(version):
 """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. """

    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()