[V3] Add tox (#1641)

* Configure tox environments for install, dev install and docs build

* Configure Travis to run tox

* Use 3.5.1 since it's our minimum supported version

* Bump lower travis build version to 3.5.2

Turns out a dependency is incompatible with 3.5.1.

* Modify Travis config to install from pipenv

* Try without skipping the lock

* Try without pip cache

* D the dev install with pipenv

* See if adding the pip cache back in breaks

* Remove the development installation

It doesn't really make any sense considering we already should be installed in develop mode, as does Travis.

* Oops, tox should go under dev packages

* Do black --check with tox

* Uncache pip again...

* Try a build matrix, and try ignoring virtualenvs

* Activate pipenv shell on travis

* Try installing prereleases

* Try the build matrix like this

* Try exclusion

* Upgrade pip

* Try this environment marker

* Back to stages...

* Try run over shell

* Try skipping the lock again

* This'll be faster but probably ignore 3.5

Because Travis

* Just manually list sources for black to check

* Magic?

* What if I told you...

That this worked perfectly on Tobotimus/Red-DiscordBot@test_travis_matrix

* It couldn't possibly be this easy

* Let's add some comments to be nice

* Let's change back to trusty just in case the stages fuck up

* Add another comment because why not

* Let's try caching pip one more time

* We don't need to whitelist these

tox.ini

@@ -0,0 +1,37 @@
+# tox (https://tox.readthedocs.io/) is a tool for running tests
+# in multiple virtualenvs. This configuration file will run the
+# test suite on all supported python versions. To use it, "pip install tox"
+# and then run "tox" from this directory.
+
+[tox]
+envlist =
+    py35
+    py36
+    docs
+    dev
+
+[testenv]
+description = Run unit tests with pytest
+extras = voice, test, mongo
+deps =
+    https://github.com/Rapptz/discord.py/archive/rewrite.zip#egg=discord.py[voice]
+    -rrequirements.txt
+commands =
+    python -m compileall ./redbot/cogs
+    pytest
+
+[testenv:docs]
+description = Attempt to build docs with sphinx-build
+whitelist_externals = sphinx-build
+basepython = python3.6
+extras = voice, docs, mongo
+commands =
+    sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out" -W -bhtml
+    sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out" -W -blinkcheck
+
+[testenv:style]
+description = Stylecheck the code with black to see if anything needs changes.
+basepython = python3.6
+extras = style
+commands =
+    black -l 99 --check generate_strings.py setup.py tests redbot