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: Deduplicate option presence testing from functional tests for ECDSA key specifications:

- Updated the functional tests that cover ECDSA key specifications.
- Dropped tests for option presence since those tests already exist in
a dedicated functional test.
- Dropped testing of invalid invocations - those are better off
covered with unit tests already.
- Reworded the tests so they make more sense.

GC-37: Deduplicate option presence testing from functional tests for RSA key specifications:

- Updated the functional tests that cover RSA key specifications.
- Dropped tests for option presence since those tests already exist in
a dedicated functional test.
- Dropped testing of invalid invocations - those are better off
covered with unit tests already.
- Reworded the tests so they make more sense.

GC-37: Added ECDSA support for issuing client certificates via client command:

- Added functional test.
- Updated unit tests.
- Expose ECDSA key specification in the client command help.

GC-37: Added ECDSA support for issuing server certificates via server command:

- Added functional test.
- Added unit tests.
- Updated existing functional test that checks for avertising of curve
support for key specification in the init command to be a bit less
fragile in case the output gets broken-up into different lines in a
slightly different location.
- Implement ability to get public key specification out of ECDSA
public key.
- Expose ECDSA key specification in the server command.
- Updated inline documentation.

GC-37: Refactor functional tests or renew command key specification handling:

- Generate the custom private keys at top of the test.
- Updated wording.
- Rename the function to explicitly reference RSA to be consistent
with the remaining tests.

GC-37: Refactor functional tests for client command key specification handling:

- Use separate test for checking use of CA hierarchy default key
specification.
- Use separate test for checking use of --key-specification option.
- Do not check public key size in certificate - this is not relevant
for this particular functional test, and there are already unit
tests that verify correct public key is used.

GC-37: Refactor functional tests for server command key specification handling:

- Use separate test for checking use of CA hierarchy default key
specification.
- Use separate test for checking use of --key-specification option.
- Do not check public key size in certificate - this is not relevant
for this particular functional test, and there are already unit
tests that verify correct public key is used.

GC-37: Added ECDSA support for initialising CA hierarchy:

- Added functional test.
- Added unit tests.
- Updated key specification parsing to support ECDSA specification
using curve name.
- Updated KeyGenerator to handle ECDSA private keys generation.
- Updated inline documentation.

GC-37: Added support for requesting custom RSA key size when renewing:

- Added functional test.
- Added unit tests.
- Updated existing functional test for renew command help to cope with
addition of one more option (output lines from help changed).
- Added new CLI option for passing-in key specification, used in
combination with the --new-private-key option.
- Renew command function now accepts key specification
parameter. Updated existing code and tests accordingly for the new
function signature.
- If key specification is not passed-in and new private key is
requested, key size is extracted from existing artefacts (e.g. it
does not use CA hierarchy's key size).

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.