Update Python version in Travis

Author: Cito

.travis.yml

@@ -18,9 +18,9 @@ matrix:
     -  env: TOXENV=pypy
        python: pypy3
     -  env: TOXENV=pre-commit
-       python: 3.6
+       python: 3.7
     -  env: TOXENV=mypy
-       python: 3.6
+       python: 3.7
 install: pip install coveralls tox
 script: tox
 after_success: coveralls