build-kivy: constrain pytest below 9.1 for the test run

Test collection failed on all matrix legs with "Failed: Marks cannot
be applied to fixtures." in common.py's async_run helper
(pytest.mark.asyncio(pytest_asyncio.fixture(func))).

Not riscv64-specific: setup.cfg's [dev] extra pins pytest>=3.6 and
pytest-asyncio!=0.11.0, both open-ended, so install_kivy_wheel dev
resolves against today's PyPI and lands on pytest 9.1+. Verified by
diffing the pytest 9.0.3 and 9.1.0 wheels directly: the same message
lives in _pytest/deprecated.py (a PytestRemovedIn8Warning) through
9.0.3, and moves to _pytest/fixtures.py as a hard fail() in 9.1.0.

A PIP_CONSTRAINT file caps pytest for the test install only, without
touching setup.cfg's own bound or the shipped wheel's dependencies
(kivy declares no pytest runtime dependency at all).

Signed-off-by: Ludovic Henry <git@ludovic.dev>

Author: luhenry

.github/workflows/build-kivy.yml

@@ -221,6 +221,19 @@ jobs:
           source .ci/ubuntu_ci.sh
           prepare_env_for_unittest
 
+      # setup.cfg's [dev] extra pins pytest>=3.6 and pytest-asyncio!=0.11.0, both with no
+      # upper bound. install_kivy_wheel below resolves that against today's PyPI, landing on
+      # pytest 9.1+, where a check that used to be a PytestRemovedIn8Warning (deprecated.py)
+      # became a hard `Failed: Marks cannot be applied to fixtures.` (fixtures.py) -- verified
+      # by diffing the pytest 9.0.3 and 9.1.0 wheels directly. common.py's own async_run
+      # decorator does pytest.mark.asyncio(pytest_asyncio.fixture(func)), exactly the pattern
+      # that now trips. A PIP_CONSTRAINT keeps pytest below that line without touching
+      # setup.cfg's own bound.
+      - name: Constrain pytest for the test run
+        run: |
+          echo 'pytest<9.1' > "${{ runner.temp }}/constraints.txt"
+          echo "PIP_CONSTRAINT=${{ runner.temp }}/constraints.txt" >> "$GITHUB_ENV"
+
       - name: Install Kivy
         run: |
           source .ci/ubuntu_ci.sh