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/application/formatters/iformatter.py

@@ -30,7 +30,7 @@ try:
     import tty, termios
 except ImportError:
     PROMPT = '--Press return to continue--'
-    def readch():
+    def readch():  # NOQA
         return sys.stdin.readline()
 else:
     PROMPT = '--Press a key to continue--'
@@ -163,7 +163,6 @@ class IFormatter(object):
             self.output(formatted)
         return formatted
 
-
     def format_obj(self, obj, alias=None):
         """
         Format an object to be human-readable.