build-kivy: tighten the pytest cap to <8, not just <9.1

<9.1 fixed the hard `Failed: Marks cannot be applied to fixtures.`
error (that's pytest 9.1.0+ only), but the resolved 9.0.x still
promotes the same underlying PytestRemovedIn9Warning to a collection
ERROR at import time -- same test_app.py collection failure, different
error class. pytest's own deprecation-escalation policy in the 9.0
pre-9.1 window apparently already enforces this one as fatal, not just
warn-worthy.

Drop the cap below the whole 8.x/9.0 cycle to pytest<8, the last major
line predating this deprecation existing at all (my earlier diff of
7.4.4's _pytest/fixtures.py and deprecated.py found neither the
warning nor the hard-error string). No change to pytest-asyncio's own
version -- pip's resolver picks whichever release is compatible with
pytest<8.

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

Author: luhenry

.github/workflows/build-kivy.yml

@@ -222,16 +222,17 @@ jobs:
           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.
+      # upper bound. install_kivy_wheel below resolves that against today's PyPI. pytest<9.1
+      # alone isn't enough: 9.0.x still promotes the underlying PytestRemovedIn9Warning
+      # (common.py's async_run does pytest.mark.asyncio(pytest_asyncio.fixture(func))) to a
+      # collection ERROR, just with a different message than 9.1's hard `Failed:` -- pytest's
+      # own pre-removal deprecation escalation, not a bug in this pin. Dropping below the
+      # whole 8.x/9.0 cycle to pytest<8 avoids both; 7.4.4's _pytest source has neither the
+      # warning nor the hard-error string at all. pytest-asyncio itself is left unconstrained;
+      # pip's resolver picks a release compatible with pytest<8.
       - name: Constrain pytest for the test run
         run: |
-          echo 'pytest<9.1' > "${{ runner.temp }}/constraints.txt"
+          echo 'pytest<8' > "${{ runner.temp }}/constraints.txt"
           echo "PIP_CONSTRAINT=${{ runner.temp }}/constraints.txt" >> "$GITHUB_ENV"
 
       - name: Install Kivy