Add tox

[adriangonz]

Resolves #1042. However, before making any changes we should discuss on #1042 if we want to introduce tox or not.

Changelog

• Add requirements-dev.txt with pytest, tox and mypy
• Add tox config to setup.cfg
• Remove all install-* targets in Makefile and the different steps in jenkins-x.yaml since the diff environments are now hanled by tox

[axsaucedo]

Very cool! It would be worth adding something in the docs to explain this, but other than that looks awesome, and very clean! Maybe just a brief on CONTRIBUTING.md?

[ukclivecox]

Looks good,

[ukclivecox]

/lgtm

[ukclivecox]

/hold

[axsaucedo]

@adriangonz I think after we add version limits + some details in contributing it should be good to go
/approved cancel
/lgtm cancel

[adriangonz]

I've just added some docs and ceilings for the versions in requirements-dev.txt @axsaucedo 👍

[axsaucedo]

Solid! Nice on @adriangonz
/hold cancel
/lgtm
/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

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