Add mypy-plugin extra dependency

use `pip install classes[mypy-plugin]` for installing additional
dependencies required by the mypy-plugin.

Author: zhukovgreen

README.md

@@ -40,6 +40,11 @@ You are also required to [configure](https://returns.readthedocs.io/en/latest/pa
 `mypy` correctly and install our plugin
 to fix [this existing issue](https://github.com/python/mypy/issues/3157):
 
+```bash
+pip install classes[mypy-plugin]
+```
+and
+
 ```ini
 # In setup.cfg or mypy.ini:
 [mypy]

pyproject.toml

@@ -48,6 +48,7 @@ _ = "returns.contrib.hypothesis._entrypoint"
 python = "^3.7"
 
 typing-extensions = ">=4.0,<5.0"
+mypy = { version = "^1.2.0", optional = true }
 
 [tool.poetry.dev-dependencies]
 anyio = "^3.6"
@@ -82,6 +83,9 @@ doc8 = "^1.0"
 m2r2 = "^0.3"
 tomlkit = "^0.11"
 
+[tool.poetry.extras]
+mypy-plugin = ["mypy"]
+
 
 [build-system]
 requires = ["poetry-core>=1.0.0"]