console: Use regular str() types for commands on Python2

Avoid using unicode() types on Python2 as it can lead to subtle
errors.

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

klippy/console.py

@@ -187,7 +187,7 @@ class KeyboardReader:
                 return None
         return line
     def process_kbd(self, eventtime):
-        self.data += os.read(self.fd, 4096).decode()
+        self.data += str(os.read(self.fd, 4096).decode())
 
         kbdlines = self.data.split('\n')
         for line in kbdlines[:-1]: