tox: force pytest version to the 4.6.4
As of today, latest version of pytest (5.0.0), released on June 28th
is freezing tox on py36 right after the test session starts.
Version 4.6.4 was released on the same date.
Signed-off-by: Armando Neto <abiagion@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>