Update to upstream 0.9.24

This MR updates the package to upstream 0.9.24, closing bugs #1058169 and #1056225.

There are also a few tweaks to the packaging:

• Add Build-Depends for python3-sphinxcontrib.trio and python3-service-identity, as required by the new upstream version.
• Modify d/rules and patch setup.py to build the included C libraries using the compiler flags recommended by Debian Hardening.
• Update the long description for python-aioquic-doc to include a brief description of the parent package.