Popen didn't gain the encoding parameter until 3.6:

https://docs.python.org/3/whatsnew/3.6.html#subprocess
On 3.5, universal_newlines is available, but encoding is not:

https://docs.python.org/3.5/library/subprocess.html
With these changes, rbt post works in a Python 3.5.6 environment. Similarly, the test suite passes for me (save 3 svn-related tests that are failing due to not have subversion installed).
I'd be happy to add some new tests if desired, but this also seems small enough in scope that it should be safe as-is.

Used rbt post in Python 2.7, 3.5, and 3.6 with these changes
rbt post works when pushing diffs with Unicode content
Test suite passes locally