Changed flake8 installation strategy, try to update it using testings/lint/flake8_requirements.txt
moved flake8_requirements to tools/lint/flake8 directory
extracted common pip invocations to _run_pip function
minor fixes
fixed calls to check_process (concatenating list to tuple)
removed update_pip function and its error message variable
if flake8 installation reports and error print pip command output - replaced out with e.output in print function
MozReview-Commit-ID: Fmq22aL8dMq
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extra : rebase_source : 2e2f942c1ffab012795c632ceacf6debf0dc1407
Francesco Pischedda