Fix incompatibility with matplotlib 3.5.0-3.9.x

[jwhiss]

Summary

This PR fixes #54 and the same issue I personally experienced with Matplotlib 3.6.3 and Python 3.11 by adding a compatibility branch in _enable_matplotlib_integration for Matplotlib versions >= 3.5.0 and < 3.10.0, using matplotlib.get_backend() without arguments.

Background

In Matplotlib 3.5.0, the private _get() function was removed and replaced by get_backend() for retrieving the current backend. get_backend() accepted no arguments through Matplotlib 3.9.x.

However, _enable_matplotlib_integration currently attempts to use matplotlib.rcParams._get("backend") for Matplotlib versions before 3.10.0. This causes the integration to fail with Matplotlib >= 3.5.0 < 3.10 because _get() is no longer available.

Changes

This change uses:

• matplotlib.get_backend(auto_select=False) for Matplotlib >= 3.10.0
• matplotlib.get_backend() for Matplotlib >= 3.5.0 and < 3.10.0
• matplotlib.rcParams._get("backend") for older Matplotlib versions

Testing

Confirmed that Matplotlib 3.5.0 and 3.6.3 do not include _get().

Tested with Matplotlib 3.6.3 and confirmed that _enable_matplotlib_integration successfully retrieves the backend without raising an AttributeError.

Included a CI test that verifies Matplotlib 3.5.0 and 3.9.9 work with the matplotlib.rcParams.get_backend() function.

Tested that the modified function works correctly with Python 3.10 and Python 3.11.

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Fixes Matplotlib backend retrieval compatibility across versions 3.5–3.9 while preserving support for older and newer releases.

Changes:

• Uses get_backend() for Matplotlib 3.5–3.9.
• Uses get_backend(auto_select=False) for Matplotlib ≥3.10.
• Retains the legacy fallback for older versions.

File	Description
matplotlib_inline/​backend_inline.py	Adds version-aware backend retrieval logic.

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