Testing Brython with CPython test suite

So far bugs in Brython have been mostly detected by users who can't make valid Python code work. A better method would be to use the test suite provided in the CPython distribution, in Lib/test. 

I have been working on the test page initialised by Billy, accessible from the url /tests/unittests/index.html. After fixing a few things to make this page work, I ran the test "test_int" ; it showed a couple of errors that I fixed in py_int.js, and I am glad to announce that this test now passes completely.

I plan to go on with this approach in the future to progress towards yet more Python compatibility, and I encourage people who want to contribute to do so, reporting the bugs they find in the Github tracker.

The main problem is currently the loading time of the page, it's because unittest imports dozens of modules.