Style files can be imported from third-party packages

Third-party packages can now distribute style files that are globally available as follows. Assume that a package is importable as import mypackage, with a mypackage/__init__.py module. Then a mypackage/presentation.mplstyle style sheet can be used as plt.style.use("mypackage.presentation").

The implementation does not actually import mypackage, making this process safe against possible import-time side effects. Subpackages (e.g. dotted.package.name) are also supported.