[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

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "411f8c9337f628fce64779f2e6ea4a59ec5b19e9cf6fcb1a264cda57805e129d"
+            "sha256": "d340e4a19777736703970e45766d05d67b973db38b87382b6ef8696cb53abb60"
         },
         "pipfile-spec": 6,
         "requires": {},
@@ -192,6 +192,13 @@
             ],
             "version": "==0.7.10"
         },
+        "appdirs": {
+            "hashes": [
+                "sha256:9e5896d1372858f8dd3344faf4e5014d21849c756c8d5701f78f8a103b372d92",
+                "sha256:d8b24664561d0d34ddfaec54636d502d7cea6e29c3eaf68f3df6180863e2166e"
+            ],
+            "version": "==1.4.3"
+        },
         "attrs": {
             "hashes": [
                 "sha256:4b90b09eeeb9b88c35bc642cbac057e45a5fd85367b985bd2809c62b7b939265",
@@ -206,6 +213,15 @@
             ],
             "version": "==2.5.3"
         },
+        "black": {
+            "hashes": [
+                "sha256:58594128efb5d21f359c8680911241985e627eb3505397f3982ffbae914d8202",
+                "sha256:dfb0e344587510ef662785f4a8991addb4017840459bb6d1d53d344ab0f10300"
+            ],
+            "index": "pypi",
+            "markers": "python_version >= '3.6'",
+            "version": "==18.4a4"
+        },
         "certifi": {
             "hashes": [
                 "sha256:13e698f54293db9f89122b0581843a782ad0934a4fe0172d2a980ba77fc61bb7",
@@ -220,6 +236,13 @@
             ],
             "version": "==3.0.4"
         },
+        "click": {
+            "hashes": [
+                "sha256:29f99fc6125fbc931b758dc053b3114e55c77a6e4c6c3a2674a2dc986016381d",
+                "sha256:f15516df478d5a56180fbf80e68f206010e6d160fc39fa508b65e035fd75130b"
+            ],
+            "version": "==6.7"
+        },
         "docutils": {
             "hashes": [
                 "sha256:02aec4bd92ab067f6ff27a38a38a41173bf01bed8f89157768c1573f53e474a6",
@@ -378,12 +401,27 @@
             ],
             "version": "==1.0.1"
         },
+        "tox": {
+            "hashes": [
+                "sha256:96efa09710a3daeeb845561ebbe1497641d9cef2ee0aea30db6969058b2bda2f",
+                "sha256:9ee7de958a43806402a38c0d2aa07fa8553f4d2c20a15b140e9f771c2afeade0"
+            ],
+            "index": "pypi",
+            "version": "==3.0.0"
+        },
         "urllib3": {
             "hashes": [
                 "sha256:06330f386d6e4b195fbfc736b297f58c5a892e4440e54d294d7004e3a9bbea1b",
                 "sha256:cc44da8e1145637334317feebd728bd869a35285b93cbb4cca2577da7e62db4f"
             ],
             "version": "==1.22"
+        },
+        "virtualenv": {
+            "hashes": [
+                "sha256:1d7e241b431e7afce47e77f8843a276f652699d1fa4f93b9d8ce0076fd7b0b54",
+                "sha256:e8e05d4714a1c51a2f5921e62f547fcb0f713ebbe959e0a7f585cc8bef71d11f"
+            ],
+            "version": "==15.2.0"
         }
     }
 }