Adding project links in PyPI and README (patch attached)

Hello and thank you very much for the amazing project.

I usually use cffi directly from PyPI, but today I tried to read it source code, so I had to spend some time trying to find out the repository URL.

Would you be interested in providing this information directly via package metadata? I believe that it is very handy for the users.

This is how this kind of links is rendered in PyPI (different project, just for illustration purposes):

I believe that it can be done by adding project_urls to the setup.cfg files (this is just one way, there are other ways, but those involve more changes).

Please find bellow a small patch implementing this change (I tried to use the collaborative features of heptapod, but I still don't know exactly how it works):

diff -r 42bc0cfb5a15 -r 5c6ee6991256 README.md
--- a/README.md Wed Jun 08 10:10:31 2022 +0200
+++ b/README.md Wed Jun 29 10:47:59 2022 +0100
@@ -10,6 +10,16 @@

 [Download page](https://foss.heptapod.net/pypy/cffi/-/tags)

+Source Code
+-----------
+
+CFFI is developed using [Mercurial SCM](mercurial-scm.org/).
+The source code is publicly available on
+[heaptapod](https://foss.heptapod.net/pypy/cffi).
+
+You can read more about how to contribute to the project on
+[PyPy's documentation](https://doc.pypy.org/en/latest/contributing.html).
+
 Contact
 -------

diff -r 42bc0cfb5a15 -r 5c6ee6991256 setup.cfg
--- a/setup.cfg Wed Jun 08 10:10:31 2022 +0200
+++ b/setup.cfg Wed Jun 29 10:47:59 2022 +0100
@@ -1,2 +1,9 @@
 [metadata]
 license_file = LICENSE
+project_urls =
+    Documentation = http://cffi.readthedocs.org/
+    Source Code = https://foss.heptapod.net/pypy/cffi
+    Issue Tracker = https://foss.heptapod.net/pypy/cffi/issues
+    Changelog = https://cffi.readthedocs.io/en/latest/whatsnew.html
+    Downloads = https://foss.heptapod.net/pypy/cffi/-/tags
+    Contact = https://groups.google.com/forum/#!forum/python-cffi