GC-45: Update copyright information:

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

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-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-28: Added initial set-up for running tests within Vagrant machine:

- Added Vagrantfile that deploys Debian 9 Stretch and provisions it.
- Added provisioning script that will set-up multiple Python versions.
- Updated development instructions to include information on how to
use Vagrant to run all the tests.
- Ignore Vagrant artifacts in gitignore.

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 basic development documentation:

- Documented development environment set-up.
- Documented how to run the tests.
- Documented how to build documentation.