chore: merge dev envs on single tox env

Co-Authored-By: Jonathan Kim <jkimbo@gmail.com>

Author: KingDarBoja

.travis.yml

@@ -11,16 +11,8 @@ python:
   - pypy3
 matrix:
   include:
-    - python: 3.6
-      env: TOXENV=flake8
-    - python: 3.6
-      env: TOXENV=black
-    - python: 3.6
-      env: TOXENV=import-order
-    - python: 3.6
-      env: TOXENV=mypy
-    - python: 3.6
-      env: TOXENV=manifest
+    - python: 3.7
+      env: TOXENV=flake8,black,import-order,mypy,manifest
 cache: pip
 install: pip install tox-travis codecov
 script: tox