[NEW] textproc/py3-sphinx: Create Python 3.x version of this port

Also make the following changes for the master textproc/py-sphinx port:

• Remove post-build introduced when port was updated to version 1.2 in r336515, but is no longer required after version 1.2.1 [1].
• Include upstream patch which fixes the bundling of grammar files [2]

[1] https://github.com/sphinx-doc/sphinx/blob/1.2.1/CHANGES#L51-L53
[2] https://github.com/sphinx-doc/sphinx/commit/b03b7e543f5e135d046d2fe407def219336c12b2

PR: 217475
PR: 217447
PR: 219833
Submitted by: dbaio
Approved by: kmoore (maintainer timeout)