what does "__requires__" mean in python?

I am newbie in python.

Could anybody answer what does __requires__ means in the following code? Why would they put __requires__ = 'flower==0.4.0' in the beginning of the file?

#!/srv/virtualenvs/zeusenv/bin/python

__requires__ = 'flower==0.4.0'
import sys
from pkg_resources import load_entry_point

sys.exit(
   load_entry_point('flower==0.4.0', 'console_scripts', 'flower')()
)