Eliminate warning in test suite

[fyellin]

Runnng the test suite from the terminal gives the warning shown below. This makes this warning go away.

I haven't yet figured out a safe way to get rid of the other warning: /Users/fy/Pycharm/wgpu-py/wgpu/gui/glfw.py:557: DeprecationWarning: There is no current event loop

================================================================================================== warnings summary ==================================================================================================
tests/test_gui_base.py::test_base_canvas_context
  /Users/fy/Pycharm/wgpu-py/.venv/lib/python3.12/site-packages/_pytest/unraisableexception.py:80: PytestUnraisableExceptionWarning: Exception ignored in: <function GPUCanvasContext.__del__ at 0x102d51ee0>
  
  Traceback (most recent call last):
    File "/Users/fy/Pycharm/wgpu-py/wgpu/_classes.py", line 517, in __del__
      self._release()
    File "/Users/fy/Pycharm/wgpu-py/wgpu/_classes.py", line 520, in _release
      self._drop_texture()
    File "/Users/fy/Pycharm/wgpu-py/wgpu/_classes.py", line 442, in _drop_texture
      if self._texture:
         ^^^^^^^^^^^^^
  AttributeError: 'GPUCanvasContext' object has no attribute '_texture'
  
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

[almarklein]

I haven't yet figured out a safe way to get rid of the other warning: /Users/fy/Pycharm/wgpu-py/wgpu/gui/glfw.py:557: DeprecationWarning: There is no current event loop

This is because of asyncio.get_event_loop() being deprecated from Python 3.12. I'll probably address that in #618