Serhiy Storchaka added the comment:
> It looks to me that interrupting fcntl can be used for implementing
blocking lock with a timeout. If automatically retry fcntl() on EINTR, this
will break such code.

If the signal handler doesn't raise an exception, the code is not reliable.
It's a common complain about the PEP 475.

If the signal handler raises an exception, it's work on Python 2.7 and on
Python 3.5 with my patch.