feat: 'notebook' extra requirement with nbformat

Simplifies `pip install radon nbformat` to `pip install radon[notebook]`.
Protects against changing underlying dependency in future.

README.rst

@@ -51,7 +51,7 @@ With Pip:
     $ pip install radon
 
 If you want to configure Radon from `pyproject.toml` and you run Python <3.11,
-you'll need the extra `toml` dependency:
+you will need to install the extra `toml` dependency:
 
 .. code-block:: sh
 
@@ -159,9 +159,11 @@ Usage with Jupyter Notebooks
 
 Radon can be used with ``.ipynb`` files to inspect code metrics for Python cells. Any ``%`` macros will be ignored in the metrics.
 
-.. note::
+You will need to install the extra `notebook` dependency:
 
-   Jupyter Notebook support requires the optional ``nbformat`` package. To install, run ``pip install nbformat``.
+.. code-block:: sh
+
+   $ pip install radon[notebook]
 
 To enable scanning of Jupyter notebooks, add the ``--include-ipynb`` flag.
 

docs/commandline.rst

@@ -42,9 +42,11 @@ Usage with Jupyter Notebooks
 
 Radon can be used with ``.ipynb`` files to inspect code metrics for Python cells. Any ``%`` macros will be ignored in the metrics.
 
-.. note::
+You will need to install the extra `notebook` dependency:
+
+.. code-block:: sh
 
-   Jupyter Notebook support requires the optional ``nbformat`` package. To install, run ``pip install nbformat``.
+   $ pip install radon[notebook]
 
 To enable scanning of Jupyter notebooks, add the ``--include-ipynb`` flag with any of the commands.
 

setup.py

@@ -1,5 +1,7 @@
 import os
+
 from setuptools import setup, find_packages
+
 import radon
 
 
@@ -27,7 +29,8 @@
           'colorama>=0.4.1;python_version>"3.4"',
       ],
       extras_require={
-          'toml': ["tomli>=2.0.1"]
+          'toml': ["tomli>=2.0.1"],
+          'notebook': ['nbformat'],
       },
       entry_points={
           'console_scripts': ['radon = radon:main'],