Make the current docs be "latest" and "0.7.3" be the redirect

That way, when people copy urls, they will remain up-to-date.
Make ./generate_redirects.py source/ dest/ work
Remove Python 3 stuff, which is no longer valid
Update README with switch to "latest" naming
Merge branch 'gh-pages' of github.com:sympy/sympy_doc into gh-pages