Travis: include mypy tests for static type checking in Python

This uses the mypy configuration contributed by @pgunn to run static
type checks on our Python codebase. For now, the tests are only run to
display potential problems, but they don't have an effect on the test
results.

.travis.yml

@@ -44,6 +44,9 @@ script:
   - cd django/applications/catmaid/static/css
   - $TRAVIS_BUILD_DIR/node_modules/.bin/csslint .
   - cd $TRAVIS_BUILD_DIR
+  # Static type checking for Python, for now only as warning and when mypy is
+  # available (which it isn't in PyPy).
+  - if [ -x "$(command -v mypy)" ]; then mypy --config-file mypy.ini django/ || true; fi
   - cd django/projects
   - python manage.py migrate --noinput
   - python manage.py collectstatic --link --noinput

django/requirements-test.txt

@@ -2,3 +2,4 @@
 django-migration-testcase==0.0.13
 selenium==3.141.0
 sauceclient==1.0.0
+mypy==0.650; platform_python_implementation != 'PyPy'