hooks : setting up flake8 and pre-commit hooks (#1681)

Small, non-functional changes were made to non-compliant files.
These include breaking up long lines, whitespace sanitation and
unused import removal.

Maximum line length in python files was set to a generous 125 chars,
in order to minimize number of changes needed in scripts and general
annoyance. The "txt" prompts directory is excluded from the checks
as it may contain oddly formatted files and strings for a good reason.

Signed-off-by: Jiri Podivin <jpodivin@gmail.com>