ENH: allow @property decorator on external ctypedef classes

Change the default language level to "3str"

Enable C++ tests in appveyor, only for Py2.7/3.7 64 bit for now.

Closes #1635.

Update URLs to https:// in docs and comments

Export "PyInit___init__" and "init__init__" as additional module init functions under Windows to make compiled packages work.

Avoid const-ness compiler warning in __Pyx_PyObject_AsWritableString

Run stackless builds in travis before the pypy builds (which take very long), and require them to work, now that they seem to succeed.