bump py37 to py38 in tox.ini
bump py37 to py38 in tox.ini

in 'victoria' cycle, we should test py38 by default.
Trivial change

Change-Id: Idd51da2a7e98e31bea6459a10f91d7eecf91d100

stackforge/monasca-statsd,stackforge/monasca-statsd