Resolve "Discontinue Python 2.7 support?": Bilby is now python3.5+

Closes #410 (closed)

Summary of changes

1. Add an check in setup.py that the python version is at least 3.5
2. Add the python_requires to the setup.py (used by pip)
3. Add an error message at import if the python version is less than 3
4. Update the documentation to indicate it is only python3 compatible
5. Update the documentation to recommend pip install