Improve compatibility with ipywidgets 8.0

Before this change pytest failed with an error:

    ==================================== ERRORS ====================================
    __________________ ERROR collecting qgrid/tests/test_grid.py ___________________
    /opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/importlib/__init__.py:126: in import_module
        return _bootstrap._gcd_import(name[level:], package, level)
    <frozen importlib._bootstrap>:1204: in _gcd_import
        ???
    <frozen importlib._bootstrap>:1176: in _find_and_load
        ???
    <frozen importlib._bootstrap>:1126: in _find_and_load_unlocked
        ???
    <frozen importlib._bootstrap>:241: in _call_with_frames_removed
        ???
    <frozen importlib._bootstrap>:1204: in _gcd_import
        ???
    <frozen importlib._bootstrap>:1176: in _find_and_load
        ???
    <frozen importlib._bootstrap>:1126: in _find_and_load_unlocked
        ???
    <frozen importlib._bootstrap>:241: in _call_with_frames_removed
        ???
    <frozen importlib._bootstrap>:1204: in _gcd_import
        ???
    <frozen importlib._bootstrap>:1176: in _find_and_load
        ???
    <frozen importlib._bootstrap>:1147: in _find_and_load_unlocked
        ???
    <frozen importlib._bootstrap>:690: in _load_unlocked
        ???
    <frozen importlib._bootstrap_external>:940: in exec_module
        ???
    <frozen importlib._bootstrap>:241: in _call_with_frames_removed
        ???
    qgrid/__init__.py:3: in <module>
        from .grid import (
    qgrid/grid.py:524: in <module>
        @widgets.register()
    E   TypeError: register() missing 1 required positional argument: 'widget'
    =============================== warnings summary ===============================
    qgrid/grid.py:30
    qgrid/grid.py:30
      /home/runner/work/qgridtrusted/qgridtrusted/qgrid/grid.py:30: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
        if LooseVersion(pd.__version__) > LooseVersion('0.20.0'):

    -- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
    =========================== short test summary info ============================

After this change that TypeError does not occur.

Author: maxwell-k

qgrid/grid.py

@@ -511,7 +511,7 @@ def stringify(x):
         return str(x)
 
 
-@widgets.register()
+@widgets.register
 class QgridWidget(widgets.DOMWidget):
     """
     The widget class which is instantiated by the ``show_grid`` method. This