travis: remove python 3.6

Python 3.6 support ended long time ago. Don't test anymore.

Author: 0xc0170

.travis.yml

@@ -245,17 +245,11 @@ matrix:
         - coverage run -a tools/project.py -S | sed -n '/^Total/p'
         - coverage html
 
-    - <<: *pytools-vm
-      name: "tools-py36"
-      env: NAME=tools-py3.6
-      python: 3.6
-
     - <<: *pytools-vm
       name: "tools-py37"
       env: NAME=tools-py3.7
       python: 3.7
 
-
     ### Extended Tests ###
     - &extended-vm
       stage: "Extended"