tests: split pyflake and flake8 tests This allow more parallelism and better skip message

flake8: run on all python file We can now run it on all file in the repository to catch potential regression.

flake8: fix error in 'setup.py' Some strange indentation in there.

flake8: fix error in 'test2rst.py' There is variosu dead code in there :-/ but we just fix flake8 complains for now.

flake8: remove "error" in documentation configuration The "error" are debatable but cheap to fix.

flake8: enable on evolve too Now that all error are fixed we can run it on all file to prevent regression.

flake8: update the code sources to fixes a horde of pyflake warnings There is many of them, they all fixed.

flake8: register 'xrange' as a builtins For some reason, flake8 thinks is is undefined.

flake8: ignore an addition rules related to import Our initialization code is too complex to make flake8 happy

flake8: document all the exception we added

README: mention the addition of the topic extensions That is probably worth noting.

tests: run pyflake on all python files We have fixed the handfull of error pyflakes found on evolve so we can setup the test to prevent regression. Next stop flake8.

serveronly: fix undefined variable This is a legit bug caught by pyflake

legacy: properly test for mercurial version in the legacy extensions Otherwise 'demandimport' would delay the raising of ImportError.

evolve: prevent global variable shadowing Small and harmless pyflake complains.