Change app in main.py to absl.app

[yx3110]

Following pysc2 commit f2f96c08c32ea92fa60485cc6d7307bc17c1c6db. The pysc2 repo does not contain app class anymore and it was replaced by Google's abseil (absl.app).
Therefore the current version of main.py no longer works. For the conveniences of new users could you please update main.py by changing
from pysc2.lib import app
import gflags as flags
to
from absl import app
from absl import flags

[Xiaowei Hu]

Thanks for your reminder.