work around pytest import file mismatch error

```
imported module 'conftest' has this __file__ attribute:
  /home/runner/work/graphviz/graphviz/tests/backend/conftest.py
which is not the same as the test file we want to collect:
  /home/runner/work/graphviz/graphviz/tests/conftest.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
```

setup.py

@@ -22,7 +22,7 @@
     python_requires='>=3.8',
     extras_require={
         'dev': ['tox>=3', 'flake8', 'pep8-naming', 'wheel', 'twine'],
-        'test': ['pytest>=7',
+        'test': ['pytest>=7,<8.1',
                  'pytest-mock>=3',
                  'pytest-cov', 'coverage'],
         'docs': ['sphinx>=5,<7', 'sphinx-autodoc-typehints', 'sphinx-rtd-theme'],