workflows: test-matplotlib: patch source, simplify python-wheels path

Add two patches for our matplotlib tests:

- One to increase a threading test timeout, helping to overcome hardware
  limitations (marked as "Inappropriate" as it's unique for our setup)
- One to backport a fix from upstream main, which is proposed for
  backporting into the 3.11.x releases but not part of 3.11.0. This pins
  pytest below version 9.1.0 due to how that version results in numerous
  errors being reported during the test runs (marked as "Backport" with
  a commit link).

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

.github/workflows/test-matplotlib.yml

@@ -47,12 +47,6 @@ jobs:
             python-version: '3.12'
 
     steps:
-      - name: Checkout python-wheels
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7.0.0
-        with:
-          path: python-wheels-repo
-          persist-credentials: false
-
       - name: Checkout matplotlib v${{ env.MPL_VERSION }}
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7.0.0
         with:
@@ -61,13 +55,25 @@ jobs:
           fetch-depth: 0
           persist-credentials: false
 
+      - name: Checkout python-wheels
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7.0.0
+        with:
+          path: python-wheels
+          persist-credentials: false
+
       - name: Set up Python ${{ matrix.python-version }}
         uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39  # v8.2.0
         with:
           python-version: ${{ matrix.python-version }}
           activate-environment: true
           enable-cache: false
 
+      # We need to patch the matplotlib source to increase a threading test
+      # timeout for the native runners
+      - name: patch matplotlib source
+        run: |
+          git apply python-wheels/patches/matplotlib/${{ env.MPL_VERSION }}/00*.patch
+
       - name: Install OS dependencies
         run: |
           echo 'Acquire::Retries "3";' | sudo tee /etc/apt/apt.conf.d/80-retries

patches/matplotlib/3.11.0/0001-TST-temporarily-pin-pytest.patch

@@ -0,0 +1,48 @@
+From afb7f27453493011e33d7bc84be964fbe84eeaf3 Mon Sep 17 00:00:00 2001
+From: Ruth Comer <10599679+rcomer@users.noreply.github.com>
+Date: Sun, 14 Jun 2026 21:04:35 +0100
+Subject: [PATCH] TST: temporarily pin pytest
+
+Upstream-Status: Backport [https://github.com/matplotlib/matplotlib/pull/31896/changes/afb7f27]
+
+Upstream devs have pinned pytest below 9.1.0 due to numerous test failures. This
+commit is only in main so far. We should backport it to avoid various
+TestSpectral errors on our end.
+
+See also: https://github.com/matplotlib/matplotlib/issues/31897
+
+Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
+---
+ environment.yml | 2 +-
+ pyproject.toml  | 2 +-
+ 2 files changed, 2 insertions(+), 2 deletions(-)
+
+diff --git a/environment.yml b/environment.yml
+index 27624759ea..057b8466fe 100644
+--- a/environment.yml
++++ b/environment.yml
+@@ -63,7 +63,7 @@ dependencies:
+   - psutil
+   - prek
+   - pydocstyle>=5.1.0
+-  - pytest!=4.6.0,!=5.4.0,!=8.1.0
++  - pytest!=4.6.0,!=5.4.0,!=8.1.0,<9.1.0
+   - pytest-cov
+   - pytest-rerunfailures
+   - pytest-timeout
+diff --git a/pyproject.toml b/pyproject.toml
+index cf0cde26cd..b63ce715ed 100644
+--- a/pyproject.toml
++++ b/pyproject.toml
+@@ -129,7 +129,7 @@ test = [
+     "certifi",
+     "coverage!=6.3",
+     "psutil; sys_platform != 'cygwin'",
+-    "pytest!=4.6.0,!=5.4.0,!=8.1.0",
++    "pytest!=4.6.0,!=5.4.0,!=8.1.0,<9.1.0",
+     "pytest-cov",
+     "pytest-rerunfailures!=16.0",
+     "pytest-timeout",
+-- 
+2.55.0
+