Add mypy check to travis CI

Also use 4 spaces for travis config file because that's how I roll.

I should add an .editorconfig here 🤔

.travis.yml

@@ -4,24 +4,27 @@ cache: pip
 sudo: required
 
 matrix:
-  fast_finish: true
-  include:
-    - python: 3.7
+    fast_finish: true
+    include:
+        - python: 3.7
 
 install:
-  - pip install -U pip
-  - pip install -U black flake8 pytest pytest-cov codecov
+    - pip install -U pip
+    - pip install -U black flake8 pytest pytest-cov codecov mypy
 
 script:
-  # Unit tests
-  - pytest --cov dragn
+    # Mypy
+    - mypy .
 
-  # Static analysis
-  - flake8 --statistics --count
+    # Unit tests
+    - pytest --cov dragn
 
-  # Code formatting
-  - black --check .
+    # Static analysis
+    - flake8 --statistics --count
+
+    # Code formatting
+    - black --check .
 
 after_success:
-  # Send coverage to Codecov
-  - codecov
+    # Send coverage to Codecov
+    - codecov