tox: Disable coverage in py3-minimal

If run after py3-full, it overwrites the full coverage info, so most lines
are incorrectly reported as uncovered in Phabricator diffs.

tox.ini

@@ -8,11 +8,11 @@ extras =
 deps =
   pytest-cov
 commands =
-  pytest --cov={envsitepackagesdir}/swh/model \
-         --doctest-modules \
-  full:    {envsitepackagesdir}/swh/model \
-  minimal: {envsitepackagesdir}/swh/model/tests/test_cli.py -m 'not requires_optional_deps' \
-         --cov-branch {posargs}
+  pytest \
+           --doctest-modules \
+  full:    --cov={envsitepackagesdir}/swh/model --cov-branch {posargs} \
+  full:    {envsitepackagesdir}/swh/model
+  minimal: {envsitepackagesdir}/swh/model/tests/test_cli.py -m 'not requires_optional_deps'
 
 [testenv:py3]
 skip_install = true