tox: Remove basepython

tox uses the python version where tox is installed by default, and we
no longer expect that the tests may run in env with python 2.

Change-Id: Ib55d7d713570a32e9e541ec23782610b9a1d6edb
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
1 file changed