Add python/external_requires keywords to setup

This should allow setuptools to write the metadata Requires-Python and
Requires-External from PEP345 to the PKGINFO file

setup.py

@@ -95,6 +95,8 @@ setup_params = dict(
             "install_requires       = setuptools.dist:check_requirements",
             "tests_require          = setuptools.dist:check_requirements",
             "setup_requires         = setuptools.dist:check_requirements",
+            "python_requires        = setuptools.dist:check_specifier",
+            "external_requires      = setuptools.dist:assert_string_list",
             "entry_points           = setuptools.dist:check_entry_points",
             "test_suite             = setuptools.dist:check_test_suite",
             "zip_safe               = setuptools.dist:assert_bool",

setuptools/dist.py

@@ -39,6 +39,22 @@ def _get_unpatched(cls):
 _Distribution = _get_unpatched(_Distribution)
 
 
+def _patch_distribution_metadata_write_pkg_file():
+    """Patch write_pkg_file to also write Requires-Python/Requires-External"""
+    original_write = distutils.dist.DistributionMetadata.write_pkg_file
+    def write_pkg_file(self, file):
+        """Write the PKG-INFO format data to a file object.
+        """
+        original_write(self, file)
+        if hasattr(self, 'python_requires'):
+            file.write('Requires-Python: %s\n' % self.python_requires)
+        if getattr(self, 'external_requires', []):
+            self._write_list(file, 'Requires-External', self.external_requires)
+
+    distutils.dist.DistributionMetadata.write_pkg_file = write_pkg_file
+_patch_distribution_metadata_write_pkg_file()
+
+
 def _patch_distribution_metadata_write_pkg_info():
     """
     Workaround issue #197 - Python 3 prior to 3.2.2 uses an environment-local
@@ -138,6 +154,18 @@ def check_requirements(dist, attr, value):
         raise DistutilsSetupError(tmpl.format(attr=attr, error=error))
 
 
+def check_specifier(dist, attr, value):
+    """Verify that value is a valid version specifier"""
+    try:
+        packaging.specifiers.SpecifierSet(value)
+    except packaging.specifiers.InvalidSpecifier as error:
+        tmpl = (
+            "{attr!r} must be a string or list of strings "
+            "containing valid version specifiers; {error}"
+        )
+        raise DistutilsSetupError(tmpl.format(attr=attr, error=error))
+
+
 def check_entry_points(dist, attr, value):
     """Verify that entry_points map is parseable"""
     try:
@@ -303,6 +331,10 @@ class Distribution(_Distribution):
                     "setuptools, pip, and PyPI. Please see PEP 440 for more "
                     "details." % self.metadata.version
                 )
+        if getattr(self, 'python_requires', None):
+            self.metadata.python_requires = self.python_requires
+        if getattr(self, 'external_requires', None):
+            self.metadata.external_requires = self.external_requires
 
     def parse_command_line(self):
         """Process features after parsing command line options"""