GC-42: Added support for Python 3.10:

- Updated documentation.
- Updated release notes.
- Updated functional tests (small tweak for help output change).
- Added signing key for the Python 3.10 and 3.11 releases.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

GC-45: Update copyright information:

- Changes to code base were previously done in 2020 as well (based on
git history).

GC-3: Add example for init command to CLI help and fix formatting:

- Updated CLI help to include example on initialising the CA
hierarchy.
- Use formatter class that preserves description formatting (otherwise
everything ends-up in a single paragrpah).

GC-3: Added dummy command for CA initialisation:

- Fixed test for the help command (help output now includes multiple
comands).
- Added dummy/skeleton implementation for init command parser.

GC-11: Added a help subcommand to the tool:

- Updated functional test for checking on the help subcommand as well.
- Implemented the 'help' subcommand.
- Updated required function signature for the subcommand_parser
decorator (functions should return ArgumentParser instance).
- Fixed a typo in inline documentation.
- Updated the test for subcommand_parser decorator to be a more robust
when checking if function registration works.

GC-11: Updated CLI to display simple usage help:

- Added terse description to the CLI.
- Implemented small mechanism for invoking callback functions attached
to parser.
- Fixed some linting issues in setup script.

GC-11: Added intial dummy CLI skeleton implementation:

- Register entry point in the package setup script.
- Implemented a very basic main function as entry point that
constructs an empty argument parser.
- Implemented functional test for testing if the CLI tool gets invoked
correctly after installation.
- Added unit tests for implemented functionality.