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 issuing client certificates:

- Added functional test.
- Added unit tests.
- Added new CLI option to the client command.
- Updated the client command to use the passed-in key specification.
- Updated existing tests to cope with changes to the client command
function signature.
- Fixed small typo in docstring for the server command.
- Fixed functional test for client command help that expected
positional argument in a specific line.

GC-16: Do not overwrite existing client artifacts:

- Added functional test for the scenario.
- Implemented unit tests that ensure client artifacts are not getting
overwritten if they already exist.
- Added check that ensures the client certificate has not been issued
previously.

GC-16: Implemented issuance of client certificates:

- Added functional test covering basic issuance of client
certificates.
- Replaced the dummy command implementation.
- Added new crypto function that can be used for issuing TLS client
certificates.
- Implemented relevant unit tests.

GC-16: Implemented scenario for issuing client certificates when hierarchy is not initialised:

- Added functional test that checks if an error is shown to user in
case the hierarchy hasn't been initialised prior to issuing a client
certificate.
- Added initial dummy client command implementation.

GC-16: Added client command parser:

- Added functional test for checking availability of client command
and its help.
- Added initial dummy implementation with parser.