Add mypy-plugin extra dependency

use `pip install classes[mypy-plugin]` for installin additional
dependencies required by the mypy-plugin.
Add mypy-plugin extra dependency

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

Author: zhukovgreen

README.md

@@ -32,9 +32,14 @@ Smart, pythonic, ad-hoc, typed polymorphism for Python.
 pip install classes
 ```
 
-You also need to [configure](https://classes.readthedocs.io/en/latest/pages/container.html#type-safety)
+You may also need to [configure](https://classes.readthedocs.io/en/latest/pages/container.html#type-safety)
 `mypy` correctly and install our plugin:
 
+```bash
+pip install classes[compatible-mypy]
+```
+and
+
 ```ini
 # In setup.cfg or mypy.ini:
 [mypy]

pyproject.toml

@@ -46,6 +46,7 @@ classifiers = [
 [tool.poetry.dependencies]
 python = "^3.7"
 typing_extensions = ">=3.10,<5.0"
+mypy = { version = "^1.2", optional = true }
 
 [tool.poetry.dev-dependencies]
 mypy = "^0.942"
@@ -71,3 +72,7 @@ tomlkit = "^0.11"
 codespell = "^2.2"
 
 phantom-types = "^1.0"
+
+[tool.poetry.extras]
+compatible-mypy = ["mypy"]
+