refcycle 0.3.0

This is a minor release that extends refcycle support through Python 3.12.

Changes

• Now supports Python 3.7 through Python 3.12. Support for earlier versions
  of Python has been dropped.
• The refcycle.version module and the refcycle.__version__ attribute
  have been removed. To get the version number of the currently installed
  package, use importlib.metadata.

Fixes

• Imports of container ABCs (like Sized, Iterable) from collections
  have been fixed to import from collections.abc instead.
• Test failures on Python 3.10 and 3.11 have been fixed.

Maintenance

• GitHub Actions workflows have been added for testing, style checks, test
  documentation build and release. The previous Travis CI-based checks have
  been removed.
• Now uses setuptools-scm for dynamic versioning.
• Copyright headers have been updated.
• Source tree now follows a src/ layout.
• The project now uses pyproject.toml rather than setup.py.
• Support for coverage via coveralls has been dropped.