Use tox to test on all envs

[pombredanne]

As suggested in aboutcode-org/commoncode#30 (comment) by @vznncv

I'm closing this pull requests since @pombredanne has proposed better solution.

@pombredanne Did you consider tox usage instead of configure scripts? Generally it does the same things (virtual environment creation for tests), but has additionally features like generative envlis that allows to test a library with different dependency versions.

@pombredanne replied:

Tox in addition would help! +1
Note that ./configure does a setup that's also useful beyond testing

So recap:

1. we should use tox
2. this may need to update on the CI setups to use tox too
3. we would still keep configure IMHO
4. we could consider tox to also test multiple versions of key dependent packages to handle issues such as Set minimal click version to 8.0 commoncode#30
5. we could consider tox to also test with the latest versions of all dependent packages