7.6.3pre4: Merge pull request #191 from mwcraig/remove-python-2-stuff

This release includes many improvements:

Improved attach_arrow
Corrections to ring
Can now have up to 32 lights (previous limit was 8)
Now uses WebGl2 which makes it possible to have up to 4.295 billion vertex objects (previous limit was 65536)
Can group radio buttons so that only one button in the group is ever highlighted
New attach_light for attaching a local_light to a moving object

There is enhanced compatibility between VPython 7 and Web VPython, because Web VPython now supports standard Python list, dictionary, and set methods

Merged PRs 180 and 183