Move pandas and matplotlib to optional extras

[araffin]

Description

Reduce the number of dependencies.

TODO:

• Open a PR in RL zoo, need to add pandas to plot dependencies

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

closes #680

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (docs/misc/changelog.md) (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have opened an associated PR on the SB3-Contrib repository (if necessary)
• I have opened an associated PR on the RL-Zoo3 repository (if necessary)
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

Note: You can run most of the checks using make commit-checks.

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[Copilot]

Pull request overview

This PR reduces Stable-Baselines3’s core dependency footprint by moving pandas and matplotlib out of install_requires and into optional extras, while adding clearer runtime errors and updating docs/tests to reflect the new optional plotting/loading behavior.

Changes:

• Move pandas/matplotlib from core dependencies to the extra extra in setup.py.
• Add explicit, user-facing ImportError messages when plotting/loading utilities are used without optional dependencies.
• Update tests and documentation to reflect the new optional dependency model; bump version to 2.9.0a2.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
setup.py	Removes pandas/matplotlib from core deps and adds them under extras_require["extra"].
stable_baselines3/common/results_plotter.py	Wraps pandas/matplotlib imports with clearer ImportErrors for plotting.
stable_baselines3/common/monitor.py	Makes pandas a lazy import in load_results() and raises a clearer ImportError if missing.
stable_baselines3/common/logger.py	Removes hard dependency on matplotlib (type-only import) and drops read_csv/read_json helpers.
tests/test_logger.py	Moves CSV/JSON reader helpers into tests and adds tests for missing pandas/matplotlib.
docs/guide/plotting.md	Documents that plotting requires pandas and matplotlib (or stable-baselines3[extra]).
docs/misc/changelog.md	Adds breaking-change notes for optional deps and moved helpers (needs minor corrections).
README.md	Updates installation/extras description and minor wording/formatting fixes.
stable_baselines3/version.txt	Bumps version from 2.9.0a1 to 2.9.0a2.

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