Deprecate the requires keyword

For runtime dependencies, install_requires should be used. For build
dependencies, a PEP 518-compliant `pyproject.toml` should be used.

Other dependencies can use extra requirements.

changelog.d/1541.deprecation.rst

@@ -0,0 +1 @@
+Remove support for "requires" parameter. 

setuptools/config.py

@@ -2,8 +2,10 @@
 import io
 import os
 import sys
+import warnings
 from collections import defaultdict
 from functools import partial
+from functools import wraps
 from importlib import import_module
 
 from distutils.errors import DistutilsOptionError, DistutilsFileError
@@ -399,6 +401,20 @@ def parse(self):
 
             section_parser_method(section_options)
 
+    def _deprecated_config_handler(self, func, msg, warning_class):
+        """ this function will wrap around parameters that are deprecated
+        
+        :param msg: deprecation message 
+        :param warning_class: class of warning exception to be raised
+        :param func: function to be wrapped around
+        """
+        @wraps(func)
+        def config_handler(*args, **kwargs):
+            warnings.warn(msg, warning_class)
+            return func(*args, **kwargs)
+
+        return config_handler
+
 
 class ConfigMetadataHandler(ConfigHandler):
 
@@ -434,7 +450,10 @@ def parsers(self):
             'platforms': parse_list,
             'keywords': parse_list,
             'provides': parse_list,
-            'requires': parse_list,
+            'requires': self._deprecated_config_handler(parse_list,
+                "The requires parameter is deprecated, please use " +
+                "install_requires for runtime dependencies.",
+                DeprecationWarning),
             'obsoletes': parse_list,
             'classifiers': self._get_parser_compound(parse_file, parse_list),
             'license': parse_file,

setuptools/tests/test_config.py

@@ -391,6 +391,23 @@ def test_classifiers(self, tmpdir):
         with get_dist(tmpdir) as dist:
             assert set(dist.metadata.classifiers) == expected
 
+    def test_deprecated_config_handlers(self, tmpdir):
+        fake_env(
+            tmpdir,
+            '[metadata]\n'
+            'version = 10.1.1\n'
+            'description = Some description\n'
+            'requires = some, requirement\n'
+        )
+
+        with pytest.deprecated_call():
+            with get_dist(tmpdir) as dist:
+                metadata = dist.metadata
+
+                assert metadata.version == '10.1.1'
+                assert metadata.description == 'Some description'
+                assert metadata.requires == ['some', 'requirement']
+
 
 class TestOptions: