Changelog:
==========
*Flag instances now raise an error if used in a bool context.
This prevents the occasional mistake of testing an instance for truthiness
rather than testing flag.value.
*absl-py no longer depends on six.
Signed-off-by: Zheng Ruoqin <zhengrq.fnst@fujitsu.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
