Forget the environment variable, and just log a warning when a metadata can't be decoded. Ref #719.

pkg_resources/__init__.py

+# coding: utf-8
+
 """
 Package resource API
 --------------------
@@ -1859,19 +1861,19 @@ class FileMetadata(EmptyProvider):
 
     def get_metadata(self, name):
         if name=='PKG-INFO':
-            env_key = 'PKG_RESOURCES_METADATA_ERRORS'
-            errors = os.environ.get(env_key, 'strict')
-            with io.open(self.path, encoding='utf-8', errors=errors) as f:
-                try:
+            with io.open(self.path, encoding='utf-8', errors="replace") as f:
                 metadata = f.read()
-                except UnicodeDecodeError as exc:
-                    # add path context to error message
-                    tmpl = " in {self.path}"
-                    exc.reason += tmpl.format(self=self)
-                    raise
+            self._warn_on_replacement(metadata)
             return metadata
         raise KeyError("No metadata except PKG-INFO is available")
 
+    def _warn_on_replacement(self, metadata):
+        replacement_char = '�'
+        if replacement_char in metadata:
+            tmpl = "{self.path} could not be properly decoded in UTF-8"
+            msg = tmpl.format(**locals())
+            warnings.warn(msg)
+
     def get_metadata_lines(self, name):
         return yield_lines(self.get_metadata(name))