[Buildbot] #3403: replace pep8+pyflakes with flake8

[Buildbot trac]

#3403: replace pep8+pyflakes with flake8
----------------------+-----------------------
Reporter:  rutsky     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  master     |   Keywords:
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 Looks like flake8 does pep8 checks and pyflakes checks, but in addition
 allows more tuning of ignored checks.

 For example, instead of doing hush_pyflakes:

 {{{
 from buildbot.process.buildrequest import BuildRequest
 _hush_pyflakes = [BuildRequest]
 }}}

 with flake8 is possible to ignore warnings from single line using `# noqa`
 directive:

 {{{
 from buildbot.process.buildrequest import BuildRequest  # noqa
 }}}

 Even author of pyflakes suggests in README:

 {{{
 If you require more options and more flexibility, you could give a look to
 Flake8 too.
 }}}

 Right now I stumbled with the issue, that it's not possible to use star
 import with pyflakes at all (yes, I need exactly star import), and AFAIK
 there is no way to disable this warning in specific file.

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Ticket URL: <http://trac.buildbot.net/ticket/3403>
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