Revert "Do not run mypy: strict py2.7 not allows"

This reverts commit d5090db

Author: penguinolog

.travis.yml

@@ -76,6 +76,14 @@ jobs:
     - pip install --upgrade bandit
     script:
     - bandit -r exec_helpers
+  - <<: *static_analysis
+    name: "MyPy"
+    install:
+    - *upgrade_python_toolset
+    - *install_deps
+    - pip install --upgrade "mypy >= 0.641"
+    script:
+    - mypy --strict exec_helpers
 
   - <<: *code_style_check
     name: "PEP8"