Warn for uppercase usage in setup.cfg metadata

Author: melissa-kun-li

setuptools/dist.py

@@ -599,6 +599,7 @@ def _parse_config_files(self, filenames=None):  # noqa: C901
 
                     val = parser.get(section, opt)
                     opt = self.dash_to_underscore_warning(opt, section)
+                    opt = self.uppercase_warning(opt, section)
                     opt_dict[opt] = (filename, val)
 
             # Make the ConfigParser forget everything (so we retain
@@ -636,6 +637,17 @@ def dash_to_underscore_warning(self, opt, section):
                 % (opt, underscore_opt))
         return underscore_opt
 
+    def uppercase_warning(self, opt, section):
+        if section in ('metadata',) and (any(c.isupper() for c in opt)):
+            lowercase_opt = opt.lower()
+            warnings.warn(
+                "Usage of uppercase key '%s' in '%s' will be deprecated in future "
+                "versions. Please use lowercase '%s' instead"
+                % (opt, section, lowercase_opt)
+            )
+            return lowercase_opt
+        return opt
+
     # FIXME: 'Distribution._set_command_options' is too complex (14)
     def _set_command_options(self, command_obj, option_dict=None):  # noqa: C901
         """