Fix!: Un-pin tox version from 3.27.1 and remove tox-pyenv

Fix!: Un-pin tox version from 3.27.1 and remove tox-pyenv

Un-pin tox version from 3.27.1 and remove tox-pyenv. Testing has
demonstrated that tox-pyenv is no longer required to obtain correct
Python runtime versions when running tests. Also, removed Python 3.8
from the VENV setup where it was being specifically requested.

Due to unpinning of the tox version, tox.ini configuration files may
need modifying to reflect a change in configuration syntax; where
whitelist_externals needs to be replaced with allowlist_externals.

Issue-ID: RELENG-4539
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>
Change-Id: Ifdd49de2a8e5054dad4a5d52125ede537049ad8e