• Python 3.14 is supported.
• Python 3.8 and 3.9 are no longer supported.
• Fix an issue where flushing using igzip_threaded caused a gzip end of stream
  and started a new gzip stream. In essence creating a concatenated gzip
  stream. Now it is in concordance with how single threaded gzip streams
  are flushed using Z_SYNC_FLUSH.
• Change build backend to setuptools-scm which is more commonly used and
  supported.
• Include test packages in the source distribution, so source distribution
  installations can be verified.
• Fix an issue where some tests failed because they ignored PYTHONPATH.
• Enable support for free-threading and build free-threaded wheels for
  CPython 3.14. Thanks to @lysnikolaou and @ngoldbaum.