Add an option to disable using trove-classifiers package

As requested in pypa/setuptools#4459, add
a VALIDATE_PYPROJECT_NO_TROVE_CLASSIFIERS environment variable that can
be used to disable using trove_classifiers package even if it is
available.  This can be used when the system features an outdated
trove_classifiers, and therefore incorrectly triggers validation error.
The change is designed to be absolutely minimal and non-intrusive.

src/validate_pyproject/formats.py

@@ -215,6 +215,9 @@ def trove_classifier(value: str) -> bool:
         """See https://pypi.org/classifiers/"""
         return value in _trove_classifiers or value.lower().startswith("private ::")
 
+    if os.getenv("VALIDATE_PYPROJECT_NO_TROVE_CLASSIFIERS"):  # pragma: no cover
+        raise ImportError()
+
 except ImportError:  # pragma: no cover
     trove_classifier = _TroveClassifier()