Cleaning up in __del__ (or any __del__, really) triggered pypy's checks
for valid C API use.

Closes: https://github.com/mclab-hbrs/DTLSSocket/issues/12

Fix compatibility with Cython 3.0

Update requirements

Pin Cython dependency to <3

DTLSSocket currently does not build with Cython 3.0.0 (released 2023-07-17).

patch to compile tinydtls on Windows (MSYS2) : autoconf is found during compilation 

- add pyproject.toml to MANIFEST.in
- bump version in setup.py

 Update to devel branch

Having those in .gitignore removes clutter at `git status`, and prevents
accidental inclusion in commits.

Since 5f32f03d, its PRNG functions are fanned out into platform specific
implementations; those are included now.