Add mypy-plugin extra dependency

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

Author: zhukovgreen

Diff for: pyproject.toml

@@ -46,6 +46,7 @@ classifiers = [
 [tool.poetry.dependencies]
 python = "^3.7"
 typing_extensions = ">=3.10,<5.0"
+mypy = { version = "^0.942", 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]
+mypy-plugin = ["mypy"]
+