Check if backend is loaded

[eyllanesc]

• tests.
• CHANGELOG.

[nicoddemus]

Thanks @eyllanesc!

Other than the small comment, can you also please:

1. Add unittests for _is_library_loaded and _get_backend_loaded?
2. Include a CHANGELOG entry.

[eyllanesc]

In addition to the changelog change, this part of the README.rst should also be rewritten.

pytest-qt/README.rst

Lines 79 to 99 in f00d7a4

	Works with either PySide6_, PySide2_, PyQt6_ or PyQt5_, picking whichever
	is available on the system, giving preference to the first one installed in
	this order:
	- ``PySide6``
	- ``PySide2``
	- ``PyQt6``
	- ``PyQt5``
	To force a particular API, set the configuration variable ``qt_api`` in your ``pytest.ini`` file to
	``pyqt6``, ``pyside2``, ``pyqt6`` or ```pyqt5``:
	.. code-block:: ini
	[pytest]
	qt_api=pyqt5
	Alternatively, you can set the ``PYTEST_QT_API`` environment
	variable to the same values described above (the environment variable wins over the configuration
	if both are set).

[nicoddemus]

Thanks @eyllanesc, and sorry for the delay!