improve coding style

flake8 reports a lot of coding style violations

flake8 | wc -l                                                                                                                                                                                                                                         
366

While some can be safely ignored (E501 line too long, E128 continuation line under-indented for visual indent, ...), others are more dangerous :

# indentation problem : 
# E111 indentation is not a multiple of four
flake8 | grep E111 | wc -l
13

flake8 does not report the pokemon exception handling, but swallowing all exceptions is not a good practice (and can be dangerous, as will handle any exception, like, for example, undefined variable (NameError))

bt.py#L27 (should except KeyError or at least Exception)
bt.py#L34 (should except KeyError or at least Exception)
bt.py#L43 (should except KeyError or at least Exception)
views.py#L60 (should except Violation.DoesNotExists or at least Exception)
views.py#L84 (should except Violation.DoesNotExists or at least Exception)

Other result from flake8 can help to keep the codebase clean (ex : unused imports or common pep8 violations)

# unused imports
flake8 | grep F401 | wc -l
35

Note : tools like autopep8 or yapf can help with common pep8 errors automagically.

I propose myself to do the dirty job, but this kind of merge request is likely to generate conflicts :)