We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: I773bc5379de0eca8357c05084519cd1c71905d1e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>