GitHub CI: update olegtarasov/get-tag to v2.1

v2 no longer works:

Error: Unable to process command '##[set-env name=GIT_TAG_NAME;]v0.5.5' successfully.
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
Warning: The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Release 0.5.4

Also: some tweaks in README and DEVELOPING

Release 0.5.3

Release 0.5.2

Release 0.5.1

GitHub workflow: publish to PyPI (not TestPyPI)

Build wheel workflow: use Poetry to build the wheel

Additionally, check whether the tag matches the package version.

Rename SupportingMatter to OutOfLineFlowables

Sphinx style sheet: Improve definition list styling

Setting label_max_width < label_min_width causes definitions with a
term exceeding label_min_width to be rendered on a separate line.

Definitions used to be rendered like this, but recent changes must
have changed this. This change was not detected because we no longer
have tests in place to detect this regression (see brechtm#169).