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.

Try Mapping protocol before Sequence protocol on getitem/setitem/delitem

Correctly remove future_directives with # cython: language_level=2

Use the same list of potential package init file names everywhere

Use the same list of potential package init file names everywhere (not just "__init__.py" but also .pxd and .pyx).
Closes #2665.

Replace MD5 file hashing by SHA-1, both because it's faster (by 25% on 64 bit Linux) and because MD5 is no longer allowed in US FIPS 140-2 environments.
Closes #2790.