Add pep8 exceptions for flake8 in setup.cfg

[mcobzarenco]

#10 suggests that the code style of gensim should ignore flake8's E501,E731,E12,W503,E402

This PR adds the rules to setup.cfg so they're documented and used by default.

[menshikh-iv]

@mcobzarenco hi, please update current flake8 run https://github.com/RaRe-Technologies/gensim/blob/develop/continuous_integration/travis/flake8_diff.sh#L143 and https://github.com/RaRe-Technologies/gensim/blob/develop/continuous_integration/travis/flake8_diff.sh#L153
As I understand, you need to specify path to config file

[menshikh-iv]

ping @mcobzarenco

[mcobzarenco]

Sorry for the delay -- ok, so:

• I removed "--ignore ..." from the arguments of flake8 in flake8_diff.sh
• I checked that flake8 uses the rules from setup.cfg when running flake8_diff.sh locally

[mcobzarenco]

One thing to keep in mind with the current way of running flake8 (only run it on the files that were changed by a PR) is that it breaks down if one tries to change the flake8 rules and/or flake8 is updated.

One fix for the former would be for flake8_diff.sh to run flake on all files if it itself has been changed.

[menshikh-iv]

@mcobzarenco please add --config argument with path explicitly for both calls.

[mcobzarenco]

flake8 seems to look for setup.cfg / tox.ini / .flake8 in the root of the repo by default (https://travis-ci.org/RaRe-Technologies/gensim/jobs/292001367 seemed to have worked), but cool I also added an explicit --config cmd line argument, (I assume flake_diff.sh gets executed from the root of the repo by travis).

[menshikh-iv]

Thanks, congratz with first contribution @mcobzarenco 👍

[mcobzarenco]

Thanks for merging!