frappy_psi.pulse: fix fatal errors

even when a module is work in progress, it should not raise
an error on import

Change-Id: I2f91301ba2b0c574ea344c36a74da0f893aa326d

frappy_psi/pulse.py

@@ -48,7 +48,7 @@ class Base(HasIO):
         return self.communicate(f'smua.measure.{cmd} = {val}')
 
 
-class Create_Pulse(Base, Readable, Writable):
+class Create_Pulse(Base, Writable):
     target = Parameter('source target', FloatRange, unit='A', readonly=False)
     width = Parameter('pulse width', FloatRange, unit="s", readonly=False)
     resistance = Parameter('resistance', FloatRange)
@@ -72,3 +72,4 @@ class Create_Pulse(Base, Readable, Writable):
 
 
 class Script(Create_Pulse):
+    pass