Move to Python3

Move spot project to python 3. The support of python 2 is removed related to the issue https://github.com/asciimoo/searx/issues/1471.

The update is mainly done with 2to3 tool and a lot of changes forthe unicode support (see https://docs.python.org/3/howto/unicode.html)

Some manuals tests are done and it allowed to fix a lot of issue about unicode but that change can lead to some regression.