travis: Drop testing Python 3.5 and 3.6

A few libraries we depend on no longer support old versions of Python
like 3.5 and 3.6. Remove testing for these old versions.

.travis.yml

@@ -201,7 +201,7 @@ matrix:
       name: "tools-py35"
       env: NAME=tools-py3.5
       language: python
-      python: 3.5
+      python: 3.7
       install:
         # Install gcc
         - source_pkg gcc
@@ -227,16 +227,6 @@ 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-vm
       stage: "Pin validation"
       name: "pinvalidate"