Add QtCore.Qt.MouseButton.MidButton alias for Qt6

[PierreRaybaut]

If QtCore.Qt.MidButton works with QtPy and Qt6, then QtCore.Qt.MouseButton.MidButton is expected to work as well.

[coveralls]

coverage: 96.781%. remained the same
when pulling bdbda26 on PR-Add-QtCore.Qt.MouseButton.MidButton-alias-for-Qt6
into 3238de7 on master.

[dalthviz]

Hi @PierreRaybaut ! Sorry for the late response and thank you for the fix! Could you also add the alias to the test at

qtpy/qtpy/tests/test_qtcore.py

Lines 172 to 190 in 3238de7

	def test_enum_access():
	"""Test scoped and unscoped enum access for qtpy.QtCore.*."""
	assert (
	QtCore.QAbstractAnimation.Stopped
	== QtCore.QAbstractAnimation.State.Stopped
	)
	assert QtCore.QEvent.ActionAdded == QtCore.QEvent.Type.ActionAdded
	assert QtCore.Qt.AlignLeft == QtCore.Qt.AlignmentFlag.AlignLeft
	assert QtCore.Qt.Key_Return == QtCore.Qt.Key.Key_Return
	assert QtCore.Qt.transparent == QtCore.Qt.GlobalColor.transparent
	assert QtCore.Qt.Widget == QtCore.Qt.WindowType.Widget
	assert QtCore.Qt.BackButton == QtCore.Qt.MouseButton.BackButton
	assert QtCore.Qt.XButton1 == QtCore.Qt.MouseButton.XButton1
	assert (
	QtCore.Qt.BackgroundColorRole
	== QtCore.Qt.ItemDataRole.BackgroundColorRole
	)
	assert QtCore.Qt.TextColorRole == QtCore.Qt.ItemDataRole.TextColorRole
	assert QtCore.Qt.MidButton == QtCore.Qt.MouseButton.MiddleButton

?

Other than that this LGTM 👍 Thanks again!