Set asyncio_mode=auto

[drew2a]

Fixes #6865

In this PR:

1. asyncio_mode has been set to auto https://pypi.org/project/pytest-asyncio/
2. All unnecessary markers have been removed
3. Requirements for tests have been updated

The test_start_core has been fixed by adding async to the function definition (which could led to a correct setting of an event loop):

@patch('tribler.core.logger.logger.load_logger_config', new=MagicMock())
@patch('tribler.core.start_core.set_process_priority', new=MagicMock())
@patch('tribler.core.start_core.check_and_enable_code_tracing', new=MagicMock())
@patch('asyncio.get_event_loop', new=MagicMock())
@patch('tribler.core.start_core.TriblerConfig.load', new=MagicMock())
@patch('tribler.core.start_core.core_session')
async def test_start_tribler_core_no_exceptions(mocked_core_session):
    # test that base logic of tribler core runs without exceptions
    run_tribler_core_session(1, 'key', Path('.'), False)
    mocked_core_session.assert_called_once()