gcode: Fix Python3 syntax error

Fix error introduced in d6c3aaad.  Also, go back to only accepting
ascii characters on input.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/gcode.py

@@ -381,8 +381,8 @@ class GCodeIO:
     def _process_data(self, eventtime):
         # Read input, separate by newline, and add to pending_commands
         try:
-            data = str(os.read(self.fd, 4096).decode('utf8'))
-        except os.error, UnicodeDecodeError:
+            data = str(os.read(self.fd, 4096).decode())
+        except (os.error, UnicodeDecodeError):
             logging.exception("Read g-code")
             return
         self.input_log.append((eventtime, data))