Use encode_array() in Drawable._get_struct_prop()

[t-wissmann]

The Xlib/protocol/rq.py file defines an helper function encode_array()
that calls tostring() or tobytes() depending on which python version is
used.

The package maintainer Christoph Egger reported that the testsuite of
the herbstluftwm project fails[1] with the error:

      def _get_struct_prop(self, pname, ptype, pstruct):
          r = self.get_property(pname, ptype, 0, pstruct.static_size // 4)
          if r and r.format == 32:
  >           value = r.value.tostring()
  E           AttributeError: 'array.array' object has no attribute 'tostring'

    /usr/lib/python3/dist-packages/Xlib/xobject/drawable.py:782: AttributeError

Backtrace:

../tests/conftest.py:624: in is_window_urgent
    hints = window.get_wm_hints()
/usr/lib/python3/dist-packages/Xlib/xobject/drawable.py:756: in get_wm_hints
    return self._get_struct_prop(Xatom.WM_HINTS, Xatom.WM_HINTS,

I assume it should be rq.encode_array() instead of .tostring() but I'm
not terribly sure.

[1] https://pbot.rmdir.de/u/4lU98VkXkdjSypXmdekh5g

[t-wissmann]

tostring() will be removed in python 3.9 https://docs.python.org/3.8/library/array.html#array.array.tostring

[codecov-io]

Codecov Report

Merging #181 (882714a) into master (4d07023) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #181   +/-   ##
=======================================
  Coverage   81.99%   81.99%           
=======================================
  Files          42       42           
  Lines        4855     4855           
=======================================
  Hits         3981     3981           
  Misses        874      874           

[vasily-v-ryabov]

Looks reasonable. Thanks! Will do the same fix for XInput.

[vasily-v-ryabov]

Released as 0.29.

[t-wissmann]

Thanks!