Adds dry-python/returns

Link: https://github.com/dry-python/returns

Currently it is the only typed monads implementation I know.

Related:
- https://returns.readthedocs.io/en/latest/
- https://github.com/dry-python/classes
- https://sobolevn.me/2019/02/python-exceptions-considered-an-antipattern
- https://sobolevn.me/2019/03/enforcing-srp
- https://github.com/typeddjango/awesome-python-typing

README.md

@@ -599,6 +599,7 @@ Inspired by [awesome-php](https://github.com/ziadoz/awesome-php).
 * [fn.py](https://github.com/kachayev/fn.py) - Functional programming in Python: implementation of missing features to enjoy FP.
 * [funcy](https://github.com/Suor/funcy) - A fancy and practical functional tools.
 * [Toolz](https://github.com/pytoolz/toolz) - A collection of functional utilities for iterators, functions, and dictionaries.
+* [returns](https://github.com/dry-python/returns) - A set of type-safe monads, tranformers, and composition utilities.
 
 ## GUI Development