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.

weboob/tools/config/yamlconfig.py

@@ -29,8 +29,8 @@ try:
     from yaml import CLoader as Loader
     from yaml import CDumper as Dumper
 except ImportError:
-    from yaml import Loader
-    from yaml import Dumper
+    from yaml import Loader  # NOQA
+    from yaml import Dumper  # NOQA
 
 from .iconfig import IConfig, ConfigError