tox: drop non-working Python 3.6 from tox tests

We no longer support Mock on RHEL 7 (`main` branch) anyway.
RHEL 8 runs on Python 3.9.

Author: praiskup

.github/workflows/fedora-tox.yml

@@ -19,7 +19,6 @@ jobs:
       matrix:
         tox_env:
           # sync with /tox.ini
-          - py36
           - py39
           - py310
           - py311

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 # sync with /.github/workflows/fedora-tox.yml
-envlist = py{36,39,310,311,312,313}
+envlist = py{39,310,311,312,313}
 skipsdist = True
 
 [testenv]