GC-45: Update copyright information:

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

GC-45: Drop support for Python 3.5, 3.6, and 3.7:

- Updated documentation, removing references to versions as being
supported.
- Switched the Vagrant test machine to Debian Bullseye (still in LTS
support) to match the main development environment.
- Updated functional tests for output changes in OpenSSL CLI from
Debian Bullseye.
- Updated Tox configuration and instructions.
- Dropped workaround for Python 3.5.
- Updated package requirements.
- Updated release notes.

GC-37: Added support for requesting custom RSA key size when initialising the CA hierarchy:

- Added functional test.
- Added unit tests.
- Added new CLI option for specifying the algorithm.
- Implemented KeyGenerator factory-like class that can be called to
generate a private key with desired specification.
- The init init function now accepts a callable that is used to
generate private keys.
- The generate_ca_hierarchy function now accepts a callable that is
used to generate private keys.
- Updated existing unit tests to cope with changes to the init
and generate_ca_hierarchy function signatures.
- Updated existing unit tests to cope with changes to existing
functionality.
- Updated existing functional tests to cope with changes in command
output.

GC-15: Prevent server command from overwriting artifacts and clean-up incorrect CLI tests:

- Added functional test for scenario where a server certificate has
already been issued.
- Updated server certificate issuance command not to overwrite the
artifacts.
- Fixed a couple of both server and init tests related to CLI parsing
to not create artifacts unless necessary, and if necessary to create
artifacts in temporary (test) directory. Otherwise intermittent
failures will happen.

GC-3: Implement option for specifying CA hierarchy depth during initialisation:

- Minor fix for option checks to make the more reliable (for short
flags).
- Added functional test for initialising a CA hierarchy with custom
depth.
- Added option for specifying the CA hierarchy depth (defaults to 1).
- Updated the init command to accept and process the CA hierarchy
depth option correctly.
- Updated function used for issuing certificates to accept list of
extensions to be added to certificate.
- Updated function used for generating the CA hierarchy to add the CA
basic constraint extension.

GC-3: Added ability to provide custom CA base name to init command:

- Added functional test to cover the new scenario (providing CA base
name).
- Updated init command to accept the CA base name to be used when
constructing the CA subject DN.
- Updated the existing tests to pass-in the CA base name explicitly.
- Updated the CLI code to allow for user to pass-in the CA base name
via option (both short and long form available).

GC-3: Updated init command to avoid errors and overwrites on already initialised directories:

- Added functional test covering the scenario where user has already
initialised the directory and is re-running the tool.
- Updated the init command to return status based on whether the
directory is already initialised or not.
- Implemented unit tests.

GC-3: Implemented CA hierarchy initialisation:

- Added functional test for initialising hierarchy on a fresh
directory.
- Implemented the init command.
- Added two new modules for working with storage and crypto.
- Added cryptography (for certificate issuance and crypto) and
python-dateutil (for better handling of certificate validities) as
installation dependencies.
- Added freezegun as test dependency (helps with testing validity
dates).
- Implemented necessary unit tests.

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.