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.

tools/pyflakes.sh

@@ -1,7 +1,12 @@
-#!/bin/bash
+#!/bin/bash -u
 cd $(dirname $0)
 cd ..
 
-# grep will return 0 only if it founds something, but our script
-# wants to return 0 when it founds nothing!
-pyflakes weboob modules contrib scripts/* | grep -v redefinition && exit 1 || exit 0
+if which flake8 >/dev/null 2>&1; then
+    set -e
+    flake8 --ignore=E,W *.py weboob modules contrib scripts/*
+else
+    # grep will return 0 only if it founds something, but our script
+    # wants to return 0 when it founds nothing!
+    pyflakes *.py weboob modules contrib scripts/* | grep -v redefinition && exit 1 || exit 0
+fi