GC-47: Add support for Python 3.13:

- Simplify the option presence checks in functional tests to avoid
having to deal with differences in output between Python 3.13 and
older versions (similar checks are already in place for things like
key specification anyway). For more details, see this commit:

https://github.com/python/cpython/commit/c4a2e8a2c5188c3288d57b80852e92c83f46f6f3

GC-44: Added support for Python 3.12:

- Updated documentation.
- Updated release notes.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

GC-43: Added support for Python 3.11:

- Updated documentation.
- Updated release notes.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

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 development-related requirements:

- Dropped ability to run setup.py test command (deprecation).
- Updated documentation for tox invocations.
- Updated all test and development-related requirements.
- Dropped use of deprecated project pytest-runner.
- Updated tox configuration (deprecations).
- Unfortunately, tox seems to mess with the COLUMNS environment
variable that breaks functional tests which depend on specific
formatting of help (breaklines included). Make sure the tests run
with column width of 80.

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-39: Added support for Python 3.9:

- Updated release notes
- Updated functional test for changes in help output in Python 3.9.
- Updated comments on public signing keys used for validating the
Python archives when using Vagrant provisioning.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

GC-33: Added support for Python 3.8:

- Updated release notes.
- Updated documentation.
- Updated Python versions used for tests.
- Updated supported Python versions in package configuration file.

GC-35: Drop support for Python 3.4:

- Updated documentation, removing Python 3.4 as supported version.
- Updated documentation to include support for Python 3.7 (this was
already supported, but documentation did not list it in a couple of
places).
- Updated Tox configuration with list of supported Python
versions.
- Build Python 3.5.x for full test against all Python versions in
order to make it independent of underlying distribution Python
version.
- Update package requirements.
- Updated release notes.

GC-14: Cleaned-up testing and development configuration and documentation:

- Enforce 100% coverage in tests (fail the tests otherwise).
- Updated development documentation to list:
- How to run tests with coverage.
- How to generate coverage in html format.
- How to build documentation.
- What tests are included when running test via tox.
- Default to running just the unit tests when not passing in any
arguments to pytest.
- Removed use of pytest-flake8.
- Do not run coverage tests by default outside of tox.
- Ensure documentation is built inside of tox environment, and that it
does not pollute the source directory.
- Have all Python interpreters specified in same section in tox
configuration file.

GC-2: Added tox configuration:

- Configured multiple environments for running tests against Python
3.4, 3.5, and 3.6.
- Configured environment for running linting tests.
- Configured environment for running documentation builds.
- Updated requirements for running linting tests.
- Updated pytest configuration to include running linting tests.
- Updated gitignore to exclude tox artefacts.