Update metadata and requirements (#12)

1. Add links on documentation and bug tracker
2. Install/update typing on python 3.7 too ( python/typing#247 )

Author: penguinolog

requirements.txt

@@ -1,2 +1,2 @@
 six >=1.10.0
-typing >= 3.6 ; python_version < "3.7"
+typing >= 3.6 ; python_version < "3.8"

setup.py

@@ -240,6 +240,10 @@ def get_simple_vars_from_src(src):
         for name, email in variables['__maintainers__'].items()
     ),
     url=variables['__url__'],
+    project_urls={
+        "Bug Tracker": "https://github.com/python-useful-helpers/advanced-descriptors/issues",
+        "Documentation": "https://advanced-descriptors.readthedocs.io/",
+    },
     version=variables['__version__'],
     license=variables['__license__'],
     description=variables['__description__'],