Bump minimum python version to 3.7.0 (#2394)

This is in anticipation of #2246, although I've written that PR to not break on 3.6, the feature itself is not usable on 3.6. So I think the best way forward is to simply require python 3.7. This also allows devs and cog creators to utilise all of the new features in 3.7, and it also updates the docs so all operating systems will have 3.7 installed.

Signed-off-by: Toby <tobyharradine@gmail.com>

.travis.yml

@@ -3,16 +3,16 @@ language: python
 cache: pip
 notifications:
   email: false
-sudo: true
 
 python:
-- 3.6.6
-- 3.7
+- 3.7.2
 env:
   global:
-    PIPENV_IGNORE_VIRTUALENVS=1
+    - PIPENV_IGNORE_VIRTUALENVS=1
   matrix:
-    TOXENV=py
+    - TOXENV=py
+    - TOXENV=docs
+    - TOXENV=style
 
 install:
 - pip install --upgrade pip tox
@@ -22,16 +22,10 @@ script:
 
 jobs:
   include:
-
-    - python: 3.6.6
-      env: TOXENV=docs
-    - python: 3.6.6
-      env: TOXENV=style
-
     # These jobs only occur on tag creation if the prior ones succeed
     - stage: PyPi Deployment
       if: tag IS present
-      python: 3.6.6
+      python: 3.7.2
       env:
         - DEPLOYING=true
         - TOXENV=py36
@@ -43,11 +37,10 @@ jobs:
           skip_cleanup: true
           on:
             repo: Cog-Creators/Red-DiscordBot
-            python: 3.6.6
             tags: true
     - stage: Crowdin Deployment
       if: tag IS present
-      python: 3.6.6
+      python: 3.7.2
       env:
         - DEPLOYING=true
         - TOXENV=py36
@@ -63,5 +56,4 @@ jobs:
         skip_cleanup: true
         on:
           repo: Cog-Creators/Red-DiscordBot
-          python: 3.6.6
           tags: true