Integrate mypy static type checker into CI

.travis.yml

@@ -1,14 +1,15 @@
+dist: xenial
 language: python
 python:
  - "3.5"
- - "3.5-dev"
  - "3.6"
- - "3.6-dev"
+ - "3.7"
 install:
- - pip install pylint~=2.3.1 requests
+ - pip install pylint~=2.3.1 requests mypy
  - pip install -r docs/requirements.txt
 script:
  - python3 -m pylint -r n -d bad-whitespace,missing-docstring,too-many-arguments,locally-disabled,line-too-long,no-else-raise,too-many-public-methods,too-many-lines,too-many-instance-attributes,too-many-locals,too-many-branches,too-many-statements,inconsistent-return-statements,invalid-name,wildcard-import,unused-wildcard-import,no-else-return,cyclic-import,unnecessary-pass instaloader
+ - python3 -m mypy -m instaloader
  - make -C docs html SPHINXOPTS="-W -n"
 deploy:
   - provider: pypi