1.7.0 fixture-config: pytest warnings

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```console
+ /usr/bin/python3 -Bm pytest -ra
=========================================================================== test session starts ============================================================================
platform linux -- Python 3.8.9, pytest-6.2.3, py-1.10.0, pluggy-0.13.1
rootdir: /home/tkloczko/rpmbuild/BUILD/pytest-fixture-config-1.7.0, configfile: setup.cfg
plugins: forked-1.3.0, shutil-1.7.0, virtualenv-1.7.0, asyncio-0.14.0, expect-1.1.0, cov-2.11.1, httpbin-1.0.0, xdist-2.2.1, flake8-1.0.7, timeout-1.4.2, betamax-0.8.1, pyfakefs-4.4.0, freezegun-0.4.2, cases-3.4.6, hypothesis-6.10.1, case-1.5.3, isort-1.3.0, aspectlib-1.5.2, flaky-3.7.0, mock-3.6.0
collected 5 items

tests/unit/test_fixture_config.py .s.s. [100%]

============================================================================= warnings summary =============================================================================
tests/unit/test_fixture_config.py:52
/home/tkloczko/rpmbuild/BUILD/pytest-fixture-config-1.7.0/tests/unit/test_fixture_config.py:52: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
Use @pytest.fixture instead; they are the same.
def yet_another_fixture():

tests/unit/test_fixture_config.py:63
/home/tkloczko/rpmbuild/BUILD/pytest-fixture-config-1.7.0/tests/unit/test_fixture_config.py:63: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
Use @pytest.fixture instead; they are the same.
def yet_some_other_fixture():

-- Docs: https://docs.pytest.org/en/stable/warnings.html
========================================================================= short test summary info ==========================================================================
SKIPPED [1] pytest_fixture_config.py:31: config variable foo missing, skipping test
SKIPPED [1] pytest_fixture_config.py:45: config variable foo missing, skipping test
================================================================= 3 passed, 2 skipped, 2 warnings in 0.05s =================================================================
```

Start with tests/unit/test_fixture_config.py, especially the fixtures at lines 52 and 63 identified in the warning output. Run pytest before and after the change; done means the five tests retain their existing results and the PytestDeprecationWarning entries no longer appear.

Written by the indexing model from the issue text.