package/python-txaio: drop python 2 support

package/python-txaio: drop python 2 support

As of upstream commit 735eb608637e7bbab4082a541ac802cc919fec22,
available since version v20.1.1, support for Python 2.x has been
dropped, and Python >= 3.5 is required. So we make python-txaio depend
on python3, and remove the python2 test of python-txaio.

We also remove the python-six dependency which is no longer used by
txaio, since upstream commit 62b0e7eaa22769687df1de8f57374cb0a42bdc4d.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>