Add explicit dependency on attrs

pytest itself dropped the dependency some time ago, but we should explicitly declare it anyway.

setup.cfg

@@ -33,6 +33,7 @@ packages=find:
 py_modules = pytest_rich
 python_requires = >=3.7
 install_requires =
+    attrs
     pytest >= 7.0
     rich
     typing_extensions; python_version < '3.8'