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

@@ -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 returns[compatible-mypy]
+```
+and
+
 ```ini
 # In setup.cfg or mypy.ini:
 [mypy]

poetry.lock

@@ -48,29 +48,30 @@ _ = "returns.contrib.hypothesis._entrypoint"
 python = "^3.7"
 
 typing-extensions = ">=4.0,<5.0"
+mypy = { version = "^1.4.0", optional = true }
 
 [tool.poetry.dev-dependencies]
 anyio = "^3.7"
 trio = "^0.22"
 attrs = "^23.1"
 httpx = "^0.24"
 
-mypy = "^1.4"
+mypy = "^1.4.0"
 wemake-python-styleguide = "^0.17"
 flake8-pytest-style = "^1.6"
 flake8-pyi = "^23.4"
-nitpick = "^0.33"
+nitpick = "^0.33.2"
 codespell = "^2.2"
-slotscheck = "^0.16"
+slotscheck = "^0.16.4"
 
 safety = "^2.3"
 
 pytest = "^7.4"
 pytest-cov = "^4.1"
 pytest-randomly = "^3.12"
-pytest-mypy-plugins = "^1.11"
+pytest-mypy-plugins = "^1.11.1"
 pytest-subtests = "^0.11"
-hypothesis = "^6.79"
+hypothesis = "^6.78"
 
 sphinx = "^5.2"
 sphinx-autodoc-typehints = "^1.23"
@@ -81,9 +82,12 @@ doc8 = "^1.0"
 m2r2 = "^0.3"
 tomlkit = "^0.11"
 
+[tool.poetry.extras]
+compatible-mypy = ["mypy"]
+
 
 [build-system]
-requires = ["poetry-core>=1.5.0"]
+requires = ["poetry-core>=1.0.0"]
 build-backend = "poetry.core.masonry.api"