chore: set py36 for special envs on travis

Author: KingDarBoja

.travis.yml

@@ -11,7 +11,7 @@ python:
   - pypy3
 matrix:
   include:
-    - python: 3.7
+    - python: 3.6
       env: TOXENV=flake8,black,import-order,mypy,manifest
 cache: pip
 install: pip install tox-travis codecov