[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

.travis.yml

@@ -5,21 +5,35 @@ notifications:
   email: false
 
 python:
-- 3.5.3
-- 3.6.1
+- 3.6.5
+- 3.5.2
+env:
+  global:
+    PIPENV_IGNORE_VIRTUALENVS=1
+  matrix:
+    # This is the environment for the main test jobs.
+    # This will run for all above specified versions of python
+    TOXENV=py
+
 install:
-- echo "git+https://github.com/Rapptz/discord.py.git@rewrite#egg=discord.py[voice]" >> requirements.txt
-- pip install -r requirements.txt
-- pip install .[test]
+- pip install --upgrade pip pipenv
+- pipenv install --skip-lock --dev
+
 script:
-- python -m compileall ./redbot/cogs
-- python -m pytest
+- pipenv run tox
 
 jobs:
   include:
+    # These jobs occur in parallel with the main test job
+    - python: 3.6.5
+      env: TOXENV=docs
+    - python: 3.6.5
+      env: TOXENV=style
+
+    # These jobs only occur on tag creation for V3/develop if the prior ones succeed
     - stage: PyPi Deployment
       if: tag IS present
-      python: 3.5.3
+      python: 3.6.5
       env:
         - DEPLOYING=true
       deploy:
@@ -35,10 +49,10 @@ jobs:
             tags: true
     - stage: Crowdin Deployment
       if: tag IS present
-      python: 3.5.3
+      python: 3.6.5
       env:
         - DEPLOYING=true
-      before_deployment:
+      before_deploy:
       - curl https://artifacts.crowdin.com/repo/GPG-KEY-crowdin | sudo apt-key add -
       - echo "deb https://artifacts.crowdin.com/repo/deb/ /" | sudo tee -a /etc/apt/sources.list
       - sudo apt-get update -qq