Update doc to use contextlib2.nullcontext

[atugushev]

nullcontext has been backported in contextlib2==0.6.0

• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features, improvements, and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Add yourself to AUTHORS in alphabetical order;

[The-Compiler]

Thank you!

[atugushev]

Thanks, @RonnyPfannschmidt, and @The-Compiler!