tests: Skip test_readme_examples if README.md is not present

This happens in wheel installs in Debian's `pybuild` environment.

Author: martinpitt

tests/test_readme_examples.py

@@ -22,6 +22,8 @@
 def readme_blocks():
     """Extract Python code blocks from README.md"""
     readme_path = Path(__file__).parent.parent / "README.md"
+    if not readme_path.exists():
+        pytest.skip(f"README.md not found at {readme_path}")
     return re.findall(r"```python\n(.*?)\n```", readme_path.read_text(), re.DOTALL)