python3-gitdb: remove python3-async dependency

The dependency on the async module was removed back in 0.6.0 (November
2014).

Whilst here, also merge the bb/inc and stop using PYTHON_PN, as we only
support Python 3.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>