Replace Flake8 with unconfigurable PEP8 + pyflakes (or turn off Flake8 flexibility).

[nejucomo]

The flake8 docs point out that it combines:

• pyflakes,
• PEP8, and
• mccabe, a flake8 plugin.

Additionally, flake8 will ignore certain lines or files of source which have magic comment strings and supports plugins. Plugins and per-project (or per-user) configurability are antithetical to onslaught's goals, so we either need to disable these features if possible, or replace flake8.

I shy away from the McCabe complexity feature, even if it were made unconfigurable, because I'd like for the onslaught criteria to be memorable and recognizable by a human. I suspect McCabe complexity thresholds are difficult to detect without using a tool.