Squashed commit of the following:

commit f25b8a4ab68165883a4d22032994e51cd4367ed9
Author: Martin <martinbrandthilde@gmail.com>
Date:   Thu Sep 5 23:32:44 2024 +0200

    add submodule tinydtls new version

commit 945a4e09b90a831713f948d567186f7d4382c891
Author: Martin <martinbrandthilde@gmail.com>
Date:   Thu Sep 5 23:31:41 2024 +0200

    removed submodule tinydtls old version

commit 1876aa3a1916fd447153d2cbd3ff934bf3d55cba
Author: Martin <martinbrandthilde@gmail.com>
Date:   Thu Sep 5 23:07:07 2024 +0200

    update submodule

commit 16f310c7337c20e8dee9523b57c39b8f91516317
Author: Martin <martinbrandthilde@gmail.com>
Date:   Thu Sep 5 22:54:26 2024 +0200

    fix build errors, add uint8_t for tinydtls/sha2.c

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

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

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

force setuptools to install Cython if it's not installed

* work relative to __file__ rather than assuming that setup.py is in the
  working directory

* update the submodule (it might be practical for some cases to make
  that conditional on the submodule directory being empty)