WIP: Add new long_description_content_type kwarg

This is used to populate the newly proposed `Description-Content-Type`
field, that is proposed in
pypa/packaging.python.org#258.

setup.py

@@ -94,6 +94,7 @@ def pypi_link(pkg_filename):
     author="Python Packaging Authority",
     author_email="[email protected]",
     long_description=long_description,
+    long_description_content_type='text/x-rst; charset=UTF-8',
     keywords="CPAN PyPI distutils eggs package management",
     url="https://github.com/pypa/setuptools",
     src_root=None,

setuptools/command/egg_info.py

@@ -397,6 +397,10 @@ def write_pkg_info(cmd, basename, filename):
         metadata = cmd.distribution.metadata
         metadata.version, oldver = cmd.egg_version, metadata.version
         metadata.name, oldname = cmd.egg_name, metadata.name
+        metadata.long_description_content_type = getattr(
+            cmd.distribution,
+            'long_description_content_type'
+        )
         try:
             # write unescaped data to PKG-INFO, so older pkg_resources
             # can still parse it

setuptools/dist.py

@@ -67,6 +67,11 @@ def write_pkg_file(self, file):
         if self.download_url:
             file.write('Download-URL: %s\n' % self.download_url)
 
+        file.write(
+            'Description-Content-Type: %s\n'
+            % (self.long_description_content_type or "UNKNOWN")
+        )
+
         long_desc = rfc822_escape(self.get_long_description())
         file.write('Description: %s\n' % long_desc)
 
@@ -340,6 +345,9 @@ def __init__(self, attrs=None):
         self.dist_files = []
         self.src_root = attrs and attrs.pop("src_root", None)
         self.patch_missing_pkg_info(attrs)
+        self.long_description_content_type = _attrs_dict.get(
+            'long_description_content_type'
+        )
         # Make sure we have any eggs needed to interpret 'attrs'
         if attrs is not None:
             self.dependency_links = attrs.pop('dependency_links', [])