Fangfrisch

- Raise Python requirement from version 3.7 to 3.8. Python 3.7 has reached
EOL in 2023 and is no longer maintained by the Python Software Foundation.

- Add CI tests using Python 3.13, drop tests for 3.7.

- Restructure Python build to ease packaging.

- Fix issue 41.

- The logic for on_update* has been largely rewritten. As a user-visible side effect, detailed
debugging information about 'on_update_*' tasks is now available.

- For improved flexibility, configuration sections may now override global 'on_update_exec' and
'on_update_timeout'. Eearlier Fangfrisch versions only examined the DEFAULTS section.

- Add internal default configurations for providers InterServer (https://www.interserver.net/)
and Fangfrisch News (https://rseichter.github.io/fangfrisch/#ffnews). As per usual, these
new providers are disabled by default.

- Python unit tests requiring network connections are now disabled by default to facilitate sandboxed
testing. Set environment variable `NETWORK_TESTS=1` to enable these tests.

- Sanesecurity (https://sanesecurity.com) provider default configuration overhaul. Switch to a less
congested mirror site, add/remove several signature URLs.

- Modernise Python build re PEP 517 (https://peps.python.org/pep-0517/).

- Support user-defined connection timeouts.

- Require SQLAlchemy version 1.4 or higher. Version 1.3 is no longer maintained by the SQLAlchemy developers.