Support the "data-requires-python" attribute in the PyPi proxy simple index page

The official PyPi repository adds a ''data-requires-python" attribute to all packages that contain a "requires_python" statement in their setup.py file.

As an example, see Django packages: [https://pypi.org/simple/django/]

Newer packages have "data-requires-python='>=3.5'" set on each link. This allows pip to find the most suitable Python package for the version of Python that's currently running.

Currently Nexus ignores this attribute. If you visit a Nexus PyPi proxy for Django, then none of the links contain a `data-requires-python` attribute and so running 'pip install django' on 2.7 will cause it to install a version of Django that only works in Python 3.

This is causing major issues for us during our switch from Python 2 to Python 3, as a lot of packages are now publishing versions that do not support Python 2. For any of these packages, when using Nexus as a PyPi proxy, will cause installations to fail.