add additional setuptools metadata pointing to the source repository

Currently, PyPI shows one link for meson -- to the homepage (the docs
site). Provide an additional convenient link to the github repo.

setup.cfg

@@ -5,6 +5,8 @@ description = A high performance build system
 author = Jussi Pakkanen
 author_email = jpakkane@gmail.com
 url = https://mesonbuild.com
+project_urls =
+    Source = https://github.com/mesonbuild/meson
 keywords =
   meson
   mesonbuild