Add mypy dependency into requirements file

Author: vyahello

README.md

@@ -12,7 +12,7 @@
 > - `pytest`
 > - `travis CI`
 
-In addition code is `fully type annotated` and covered with static code analysis tools (`black`, `flake8`).
+In addition code is `fully type annotated` and covered with static code analysis tools (`black`, `flake8` and `mypy`).
 
 ## Run automated tests
 From the root directory of your shell run following commands:

requirements-dev.txt

@@ -4,4 +4,5 @@ pytest-clarity==0.2.0a1
 pytest-metadata==1.8
 black==19.10b0
 coveralls==1.8.2
-flake8==3.7.9
+flake8==3.7.9
+mypy==0.740