FEniCS 2017.2 will be the last release that is compatible with Python 2 (python). FEniCS 2018.1 will support Python 3 only (python3) (http://www.python3statement.org/).
DOLFIN 2018.1, a C++ library, will be exposed to Python 3 using pybind11, rather than SWIG. There will be some minor interface changes and 3rd party libraries should be tested and updated. The easiest way to test with something similar to 2018.1 is to use our Docker nightly image: