frappy_psi.mercury: communication needs latin-1 encoding

this means also StringType(isUTF8=True)

frappy_psi/mercury.py

@@ -64,6 +64,11 @@ fast_slow = Mapped(ON=0, OFF=1)  # maps OIs slow=ON/fast=OFF to sample_rate.slow
 class IO(StringIO):
     identification = [('*IDN?', r'IDN:OXFORD INSTRUMENTS:*')]
     timeout = 5
+    encoding = 'latin1'
+
+    @Command(StringType(), result=StringType(isUTF8=True))
+    def communicate(self, cmd, noreply=False):
+        return super().communicate(cmd, noreply)
 
 
 class MercuryChannel(HasIO):