Brython-3.7.0rc2

This is the second, and probably last, release candidate of Brython 3.7.0, the first one based on the same Python version.

Apart from the usual bug fixes, this release focuses on the support of modules copied from CPython 3.7.0 standard library. Support of the standard "traceback" module required the most changes. Many other modules implemented in C in CPython and that were implemented in pure Python in previous versions have been implemented in Javascript in this version.