SafeConfigParser has been deprecated since Python 3.2 and was removed
in Python 3.12 entirely.
Per the release notes [1] it is recommended to use the ConfigParser
class now.

Additionally, the readfp method has been removed in favor of read_file.

[1] - https://docs.python.org/3.12/whatsnew/3.12.html#configparser

This included a number of changes related to the CPython API.

I additionally bumped the supported python versions in CI.

Constrained the Cython version to be less than 3.0, but greater than
0.29.34.
Updated the setup.py to account for the updated cythonize parameters.
Also made the appveyor version PEP440 compatible

Rebuilt the project with the latest version of Cython and unbound the
upper limit of Cython. This was a big jump, but now it a good time to
make it.

This commit adds all relevant files from the libssh2 repo, as of commit
631e773, with some additional attribution and licenses.

while specified in the gcc options, this causes the extensions to not
link against libssh2 any more, with predictable outcomes
(i.e. runtime linking errors)