[build] Move tests to work with py37

Due to a dependency that uses py37 primatives we need to move the tests
to that version. There are no features in python3.7 being used in the
client bindings.

.travis.yml

@@ -85,25 +85,25 @@ matrix:
       <<: *java
     - env: TOXENV=flake8
       <<: *python
-      python: 3.6
+      python: 3.7
     - env: TOXENV=docs
       <<: *python
-      python: 3.6
+      python: 3.7
     - env: BZL="test //py:unit"
       <<: *python
-      python: 3.6
+      python: 3.7
     - env: CHROME=1 BZL="test //py:test-chrome"
       <<: *python
       <<: *chrome
-      python: 3.6
+      python: 3.7
     - env: MARIONETTE=1 BZL="test --jobs 1 //py:test-remote"
       <<: *python
       <<: *firefox-nightly
-      python: 3.6
+      python: 3.7
     - env: MARIONETTE=1 BZL="test //py:test-firefox"
       <<: *python
       <<: *firefox-nightly
-      python: 3.6
+      python: 3.7
     - env: TASK=//rb:unit-test
       <<: *ruby
     - env: TASK=//rb:unit-test