Avoid using pip 2020-resolver (#25)

It seams that our tox -e py36-devel is also affected by the endless
loop install bug from the new resolver, we are forced to disable it,
at least for this job.

Example: https://github.com/ansible-community/molecule-podman/pull/23/checks?check_run_id=1458663833
Related: pypa/pip#6536
Related: Textualize/rich#446

tox.ini

@@ -33,8 +33,12 @@ setenv =
     PYTHONUNBUFFERED=1
     # new resolve a must or test extras will not install right
     PIP_USE_FEATURE=2020-resolver
+    # endless loop bug on devel
+    devel: PIP_USE_FEATURE=
     MOLECULE_NO_LOG=0
 deps =
+    # https://github.com/willmcgugan/rich/issues/446
+    py36-devel: dataclasses==0.7
     devel: ansible>=2.10.0a2,<2.11
     py{36,37,38,39}: molecule[ansible,test]
     py{36,37,38,39}-{devel}: git+https://github.com/ansible-community/molecule.git@master#egg=molecule[test]