Python 2 to 3.7 compatibility release

The main point of this release is to point to a version that remains Python 2 compatible (and also works for Python 3, up to version 3.7). Forthcoming versions will likely not support Python 2, in order to better support Python 3.8+ versions. (particles used time.clock() to measure the CPU time of a SMC run, but this feature has been removed from 3.8, and replacements are available only since version 3.3).

Unless you really need Python 2 for some reason (in that case I'd be happy to hear why), do not bother about this release, and simply use the latest version of the master branch.