Add new long_description_content_type kwarg

This is used to populate the new `Description-Content-Type` field.

`Description-Content-Type` is described at
pypa/packaging.python.org#258

setup.py

@@ -95,6 +95,7 @@ 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

@@ -599,6 +599,10 @@ 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

@@ -55,6 +55,13 @@ def write_pkg_file(self, file):
     if self.download_url:
         file.write('Download-URL: %s\n' % self.download_url)
 
+    long_desc_content_type = getattr(
+        self,
+        'long_description_content_type',
+        None
+    ) or 'UNKNOWN'
+    file.write('Description-Content-Type: %s\n' % long_desc_content_type)
+
     long_desc = rfc822_escape(self.get_long_description())
     file.write('Description: %s\n' % long_desc)
 
@@ -312,6 +319,9 @@ 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', [])