Use flake8 if available instead of pyflakes

With flake8, we can check for more issues and ignore those who are not
real issues.

This allowed me to find genuine errors in:
- modules/boursorama/pages/account_history.py
- modules/ing/pages/login.py
- weboob/tools/application/qt/qt.py
I left one in weboob/tools/browser/browser.py for the time being.

Some PEP8 fixes on other files.

contrib/hds/export.py

@@ -21,7 +21,7 @@
 try:
     import sqlite3 as sqlite
 except ImportError, e:
-    from pysqlite2 import dbapi2 as sqlite
+    from pysqlite2 import dbapi2 as sqlite  # NOQA
 
 from weboob.core import Weboob
 from weboob.core.modules import ModuleLoadError
@@ -30,6 +30,7 @@ import logging
 level = logging.DEBUG
 logging.basicConfig(stream=sys.stdout, level=level)
 
+
 def main(filename):
     weboob = Weboob()
     try: