Borgmatic

* 390: Add link to Hetzner storage offering from the documentation.
* 398: Clarify canonical home of borgmatic in documentation.
* 406: Clarify that spaces in path names should not be backslashed in path names.
* 423: Fix error handling to error loudly when Borg gets killed due to running out of memory!
* Fix build so as not to attempt to build and push documentation for a non-main branch.
* "Fix" build failure with Alpine Edge by switching from Edge to Alpine 3.13.
* Move borgmatic IRC channel from Freenode to Libera Chat due to Freenode takeover drama.
IRC connection info: https://torsion.org/borgmatic/#issues

* 373: Document that passphrase is used for Borg keyfile encryption, not just repokey encryption.
* 404: Add support for ruamel.yaml 0.17.x YAML parsing library.
* Update systemd service example to return a permission error when a system call isn't permitted
(instead of terminating borgmatic outright).
* Drop support for Python 3.5, which has been end-of-lifed.
* Add support for Python 3.9.
* Update versions of test dependencies (test_requirements.txt and test containers).
* Only support black code formatter on Python 3.8+. New black dependencies make installation
difficult on older versions of Python.
* Replace "improve this documentation" form with link to support and ticket tracker.

* Fix for previous release with incorrect version suffix in setup.py. No other changes.

* 341: Add "temporary_directory" option for changing Borg's temporary directory.
* 352: Lock down systemd security settings in sample systemd service file.
* 355: Fix traceback when a database hook value is null in a configuration file.
* 361: Merge override values when specifying the "--override" flag multiple times. The previous
behavior was to take the value of the last "--override" flag only.
* 367: Fix traceback when upgrading old INI-style configuration with upgrade-borgmatic-config.
* 368: Fix signal forwarding from borgmatic to Borg resulting in recursion traceback.
* 369: Document support for Borg placeholders in repository names.

* 347: Add hooks that run for the "extract" action: "before_extract" and "after_extract".
* 350: Fix traceback when a configuration directory is non-readable due to directory permissions.
* Add documentation navigation links on left side of all documentation pages.
* Clarify documentation on configuration overrides, specifically the portion about list syntax:
http://torsion.org/borgmatic/docs/how-to/make-per-application-backups/#configuration-overrides
* Clarify documentation overview of monitoring options:
http://torsion.org/borgmatic/docs/how-to/monitor-your-backups/

* 300: Add "borgmatic export-tar" action to export an archive to a tar-formatted file or stream.
* 339: Fix for intermittent timing-related test failure of logging function.
* Clarify database documentation about excluding named pipes and character/block devices to prevent
hangs.
* Add documentation on how to make backups redundant with multiple repositories:
https://torsion.org/borgmatic/docs/how-to/make-backups-redundant/