One of suggestions discussed in the mailling thread  is to decouple
linters from test-requirements.txt. We uses specific versions of linters
(hacking, flake8, pylint and bandit) and they are potentially incompatbile
with other libraries in test-requirements.txt.
One example is bandit incompatibility detected by the new pip resolver.
This commit moves linters to tox.ini and introduces a new tox envdir
shared by linter related tox envs.
Note that we have a unit test for a local hacking rule so hacking needs
to be installed as part of the default dependencies. We can have it in
test-requirements.txt but I think it is better to have it in tox.ini
as hacking is a part of linters and other linter dependencies are
declared in tox.ini now.