[sphinx] Set version using pkg_resources
[sphinx] Set version using pkg_resources

Based on the latest recommendation from `setuptools_scm`, but using the
full version, as before:
https://github.com/pypa/setuptools_scm/issues/84

I removed the variable comments because they are no longer up to date.
They can be seen in the official documentation.

brainiak/brainiak,CameronTEllis/brainiak,CameronTEllis/brainiak,TuKo/brainiak,brainiak/brainiak,TuKo/brainiak,yidawang/brainiak,yidawang/brainiak,mihaic/brainiak,TuKo/brainiak,yidawang/brainiak,lcnature/brainiak,mihaic/brainiak,mihaic/brainiak,IntelPNI/brainiak,lcnature/brainiak,lcnature/brainiak,brainiak/brainiak,IntelPNI/brainiak,IntelPNI/brainiak,CameronTEllis/brainiak