Handle custom build_py inheriting from distutils

According to #2849, some projects, including important data-science
packages rely on `distutils` when creating custom commands, instead of
extending the ones provided by setuptools.

This change should accomodate this use case, while also warning the
users to migrate.

setuptools/command/egg_info.py

@@ -618,7 +618,15 @@ def _safe_data_files(self, build_py):
         Therefore, avoid triggering any attempt of
         analyzing/building the manifest again.
         """
-        return build_py.get_data_files_without_manifest()
+        if hasattr(build_py, 'get_data_files_without_manifest'):
+            return build_py.get_data_files_without_manifest()
+
+        log.warn(
+            "Custom 'build_py' does not implement "
+            "'get_data_files_without_manifest'.\nPlease extend command classes"
+            " from setuptools instead of distutils."
+        )
+        return build_py.get_data_files()
 
 
 def write_file(filename, contents):