Add python linting using flake8

[ashimaathri]

This helps keep the code consistent and provides an extra check for the code.
Some type of errors that it caught were:

1. An error that was introduced in a previous PR Use importlib in place of backports.test.support #375 (src/saml2/s_utils.py:384:43: F821 undefined name 'sha')
2. Usage of unicode and long which is not supported anymore in python3
3. Helped identify unused imports

Improves code quality by 0.51% 😸 https://landscape.io/github/rohe/pysaml2/pulls/378
The 'new' issues reported by landscape are not new at all. I think after the refactor, it began detecting them.