I have joined the group of projects aiming at dropping Python 2 support by 2020 (http://www.python3statement.org/).
rpy2 has been primarily developed for Python 3, with compatibility with Python 2 worked out from the Python 3 code base, for quite while now so the path to Python 3-only(*) support will be through:
adding Python 3-only features. Some of this might already be present (support for strings and encodings in better when running rpy2 with Python 3).
cleaning the current code (I can wait to see all the pragmas with conditional blocks in the C code go)
rpy2-2.9.0, that I hope to release next year, would be a potential candidate to be the first Python 3-only release. rpy2-2.8.x could remain on extended support.
(* at the moment this would be Python >= 3.5)
Any thought ?