Pydantic-argparse

Changes
- Implements handling for environment variables using `pydantic.BaseSettings` (see: 41)
- Reworks the display of validation and error messages (see: 41)
- Refactors the `utils` module (see: 41)
- Overhauls development tooling (`flake8` -> `ruff`, upgrades, etc.) (see: 41)
- Updates documentation (see: 41)
- Updates `README.md` to use raw Github logo link, rather than local path. (see: 40)
- Reworks Github workflows to use `actions/setup-python` inbuilt caching (see: 43)

Closes
* Closes (34): Required field marked as optional
* Closes (42): Investigate Python Github actions built-in caching

Changes
- Added project logo using Dall-E-2
- Fixed Coveralls CI/CD badges
- Added support for Python 3.7 and 3.8 - by kytta

Closes
* Closes (12): Add project logo
* Closes (33): Coveralls badge broken
* Closes (35 ): Consider supporting older Python versions?

Changes
- Explicit package re-exports using `__all__` (see: 23)
- Renamed `__version__.py` to `__metadata__.py` (metadata items still re-exported in base package)
- Updated linter from `pylint` to `flake8` with plugins (see: 13)
- Updated codebase to pass new linting
- Upgraded `poetry.lock` lockfile dependencies for development (see: 19)
- Updated codebase to match latest type-checking and linting versions
- Added Github Actions CI for linting, type-checking, unit-testing and coverage (see: 7)
- Added `README.md` badges for the above Github Actions

Closes
* Closes (23): Fix PyLance Private Import Message
* Closes (13): Enhancement: Investigate `flake8` and its plugins
* Closes (19): Enhancement: Investigate removal of `poetry.lock` file from version control
* Closes (7): Enhancement: Add `lint`, `type check` and `unit test` merge checks

Changes
- No longer explicitly sets default values for arguments not provided by the user
- Uses `argparse.SUPPRESS` to ignore missing values, and missing values are set by the `pydantic` model
- This allows the correct usage of `model.__fields_set__` and `model.json(exclude_unset=True)`, etc.

Closes
* Closes (18 ): Do not set values for arguments not provided by the user

Changes
- Added hosted documentation at: at https://pydantic-argparse.supimdos.com/
- Added examples
- Docstring and Comment Overhaul
- Removed `typing-inspect` dependency
- Improved argument type determination behind the scenes
- Improved linting and style-guide conformance
- Fixed broken `metavars` for variadic argument types
- Added `alias` feature for arguments
- Added `py.typed` PEP-561 marker

Closes
* Closes (8): Adds full first pass of hosted documentation
* Closes (10): Change `json` parser to generic `mapping` parser
* Closes (11 ): Change `container` parser to generic `sequence` parser

Changes
* **Feature (9)**: Adds sub-command parsing via nesting `pydantic.BaseModel` objects (closes 5).