Py3AMF 0.9.0

• Test CPython 3.11 through 3.14 while keeping installation open for older Python 3 versions where possible.
• Keep AMF0, AMF3, core remoting, the WSGI gateway, and the pure Python runtime path as the supported 0.9.0 surface.
• Keep pyamf.adapters helper modules available, while removing legacy framework adapter and gateway tests from the default suite.
• Drop official support for Jython and Cython extension builds. Existing Cython sources remain in the source tree for compatibility and reference, but Cython is no longer installed as a development dependency and extension builds are disabled during installation.
• Drop official support for Twisted, Django, and Google App Engine gateway integrations.
• Drop official support for Django, SQLAlchemy, Elixir, and Google App Engine adapter integrations. Existing adapter modules remain available as helper and legacy modules, but framework-specific object conversion should happen at the application layer.
• Split runtime and full test dependencies into requirements.txt and test-requirements.txt.
• Remove deprecated setuptools test_suite and tests_require metadata, and remove the deprecated license classifier.
• Add PEP 517 build metadata and fix source distribution metadata so runtime dependencies are installed correctly from both wheels and sdists.
• Add GitHub Actions coverage for the supported CPython 3.11 through 3.14 test matrix.
• Refresh installation, architecture, community, and tutorial documentation to advertise only the supported 0.9.0 runtime surface.