Commit c87624d ('Add Randr error classes and register them during
init') introduces three new error classes to xrandr, but makes them
inherit from Exception.

Let all error classes in randr inherit from Xerror, like all other code
expects.

Fixes the traceback

```
...
  File "/tmp/python-env/lib/python3.8/site-packages/Xlib/display.py", line 182, in sync
    self.get_pointer_control()
  File "/tmp/python-env/lib/python3.8/site-packages/Xlib/display.py", line 833, in get_pointer_control
    return request.GetPointerControl(display = self.display)
  File "/tmp/python-env/lib/python3.8/site-packages/Xlib/protocol/rq.py", line 1369, in __init__
    self.reply()
  File "/tmp/python-env/lib/python3.8/site-packages/Xlib/protocol/rq.py", line 1381, in reply
    self._display.send_and_recv(request = self._serial)
  File "/tmp/python-env/lib/python3.8/site-packages/Xlib/protocol/display.py", line 612, in send_and_recv
    gotreq = self.parse_response(request)
  File "/tmp/python-env/lib/python3.8/site-packages/Xlib/protocol/display.py", line 719, in parse_response
    gotreq = self.parse_error_response(request) or gotreq
  File "/tmp/python-env/lib/python3.8/site-packages/Xlib/protocol/display.py", line 745, in parse_error_response
    req = self.get_waiting_request(e.sequence_number)
AttributeError: 'BadRRModeError' object has no attribute 'sequence_number'
```

fixes #241

randr: inherit XError in error classes

…o dev requirements