simple_pid/PID.py: Fix Travis CI error.

Author: IhorNehrutsa

simple_pid/PID.py

@@ -100,7 +100,9 @@ def __call__(self, input_, dt=None):
         elif dt <= 0:
             raise ValueError('dt has negative value {}, must be positive'.format(dt))
 
-        if (self.sample_time is not None) and (dt < self.sample_time) and (self._last_output is not None):
+        if (self.sample_time is not None) \
+        and (dt < self.sample_time) \
+        and (self._last_output is not None):
             # only update every sample_time seconds
             return self._last_output