Mimic pytest's project_urls configuration (#331)

Adds Documentation, Changelog, Source and Tracker urls.

Author: koutoftimer

setup.py

@@ -37,4 +37,10 @@
         "Programming Language :: Python :: 3 :: Only",
         "Topic :: Software Development :: Testing",
     ],
+    project_urls={
+        "Documentation": "https://pytest-mock.readthedocs.io/en/latest/",
+        "Changelog": "https://pytest-mock.readthedocs.io/en/latest/changelog.html",
+        "Source": "https://github.com/pytest-dev/pytest-mock/",
+        "Tracker": "https://github.com/pytest-dev/pytest-mock/issues",
+    },
 )