2.4: pytest warning

Nobody has claimed this yet.

```console
+ /usr/bin/pytest -ra
=========================================================================== test session starts ============================================================================
platform linux -- Python 3.8.12, pytest-6.2.5, py-1.10.0, pluggy-0.13.1
benchmark: 3.4.1 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
Using --randomly-seed=661925073
rootdir: /home/tkloczko/rpmbuild/BUILD/contextvars-2.4, configfile: pytest.ini, testpaths: tests
plugins: forked-1.3.0, shutil-1.7.0, virtualenv-1.7.0, expect-1.1.0, flake8-1.0.7, timeout-1.4.2, betamax-0.8.1, freezegun-0.4.2, aspectlib-1.5.2, toolbox-0.5, rerunfailures-9.1.1, requests-mock-1.9.3, cov-2.12.1, flaky-3.7.0, benchmark-3.4.1, xdist-2.3.0, pylama-7.7.1, datadir-1.3.1, regressions-2.2.0, cases-3.6.3, xprocess-0.18.1, black-0.3.12, anyio-3.3.0, asyncio-0.15.1, trio-0.7.0, subtests-0.5.0, isort-2.0.0, hypothesis-6.14.6, mock-3.6.1, profiling-1.7.0, randomly-3.8.0, Faker-8.12.1, nose2pytest-1.0.8, pyfakefs-4.5.1, tornado-0.8.1, twisted-1.13.3
collected 21 items

tests/test_basics.py .....................

============================================================================= warnings summary =============================================================================
../../../../../usr/lib/python3.8/site-packages/_pytest/config/__init__.py:1183
/usr/lib/python3.8/site-packages/_pytest/config/__init__.py:1183: PytestDeprecationWarning: The --strict option is deprecated, use --strict-markers instead.
self.issue_config_time_warning(

-- Docs: https://docs.pytest.org/en/stable/warnings.html
====================================================================== 21 passed, 1 warning in 0.54s =======================================================================
pytest-xprocess reminder::Be sure to terminate the started process by running 'pytest --xkill' if you have not explicitly done so in your fixture with 'xprocess.getinfo().terminate()'.
```
And here it tis the fix:
```patch
--- a/pytest.ini~ 2019-04-01 15:41:13.000000000 +0100
+++ b/pytest.ini 2021-09-04 10:22:39.096735323 +0100
@@ -1,4 +1,4 @@
[pytest]
-addopts = --capture=no --assert=plain --strict --tb native
+addopts = --capture=no --assert=plain --strict-markers --tb native
testpaths = tests
filterwarnings = default
```

The warning is reported while running pytest and the relevant configuration is pytest.ini; start by running /usr/bin/pytest -ra to reproduce it. Check the addopts setting against the warning and the proposed change in the issue, then rerun the 21 tests. Done means the tests pass without the deprecated-option warning.

Written by the indexing model from the issue text.