Merge pull request #70 from makermelissa/master

Fix error caused by non-existent variable_frequency property

Author: tekktrik

adafruit_motor/stepper.py

@@ -114,12 +114,13 @@ def __init__(
             self._coil = (ain2, bin1, ain1, bin2)
             for i in range(4):
                 if self._coil[i].frequency < 1500:
-                    if not self._coil[i].variable_frequency:
+                    try:
+                        self._coil[i].frequency = 2000
+                    except AttributeError as err:
                         raise ValueError(
                             "PWMOut outputs must either be set to at least "
                             "1500 Hz or allow variable frequency."
-                        )
-                    self._coil[i].frequency = 2000
+                        ) from err
             if microsteps < 2:
                 raise ValueError("Microsteps must be at least 2")
             if microsteps % 2 == 1: