Drop support for Python<3.6

[thedrow]

Python 2.7 and Python 3.5 has already reached their end of life.
Python 3.6 is nearing its own end of life and is currently only receiving security patches.

Dropping support would make maintaining the library easier.

[poppyred]

@thedrow This is a good idea. The next release will remove the 3.5 support