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

[Buildbot trac]

#3403: replace pep8+pyflakes with flake8
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Reporter:  rutsky     |       Owner:
    Type:  undecided  |      Status:  new
Priority:  major      |   Milestone:  undecided
 Version:  master     |  Resolution:
Keywords:             |
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Comment (by rutsky):

 Is "do only pep8 check" step is required?
 It's definitely useful, but can we live with only "do pep8+pyflakes" and
 "ignore" pyflakes output?

 Since I rely on ignoring of PEP8 in  Slave->Worker Renaming project
 (https://github.com/buildbot/buildbot/pull/1943) I've done some changes to
 support flake8 in validate.sh and in Travis:
 https://github.com/rutsky/buildbot/commit/00c1adf4eaea07168012af17e0711975286a9419
 https://github.com/rutsky/buildbot/commit/af446532070560f6921c0b2b59eacb4e520a5f66

 However I didn't removed yet PEP8 step and now PEP8 config is duplicated
 (in `common/flake8rc` and `common/pep8rc`).

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Ticket URL: <http://trac.buildbot.net/ticket/3403#comment:1>
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