workflows: add py3.6 to tox workflow, drop py3.8

Tox workflow is missing py3.6 and py3.8. Py3.6 can be installed but
py3.8 is no longer available so I propose we drop it.

As per tox FAQ we need to set `requires = virtualenv<20.22.0` as never
virtualenv version dropped support for version <=py3.6.

https://tox.wiki/en/latest/faq.html#testing-end-of-life-python-versions

Author: tomasmatus

.github/workflows/tox.yaml

@@ -19,7 +19,7 @@ jobs:
     steps:
       - name: Install tox dependencies
         run: |
-          dnf install -y git-core tox util-linux
+          dnf install -y git-core tox util-linux python3.6
           useradd tox
 
       # https://github.blog/2022-04-12-git-security-vulnerability-announced/

pyproject.toml

@@ -197,8 +197,9 @@ legacy_tox_ini = """
 [tox]
 envlist = lint,pytest
 isolated_build = True
+requires = virtualenv<20.22.0
 labels =
-  venv = py3{6,8,9,10,11,12,13}-pytest
+  venv = py3{6,9,10,11,12,13}-pytest
 
 # The default test environments use system packages and never PyPI.
 [testenv:{lint,pytest}]