Essentials-openapi

- Adds support for Python 3.12.
- Adds `MarkupSafe` among required dependencies (and not optional).
- Adds support for latest function `model_dump` in Pydantic 2 (for examples
defined using Pydantic models).
- Upgrades development dependencies.
- Fix bug with missing items entry 36 by mh7d and mh-at-fujitsu

- Fixes example generation breaks on explicitly enumerated array elements 31,
by jjedele.

- Fixes [Markdown missing a newline after simple response types](https://github.com/Neoteroi/essentials-openapi/issues/27).
- Fixes [Empty header when operations don't have any tag.](https://github.com/Neoteroi/essentials-openapi/issues/28).
- When operations don't have any tag, the `h2` element
- Improves tests.
- Adopts `pyproject.toml`.
- Workflow maintenance.

- Fixes a bug happening when trying to serialize examples in JSON, when they
contain datetimes and are provided in YAML;
([bug report](https://github.com/Neoteroi/mkdocs-plugins/issues/35)).
- Fixes a bug related to missing resolution of references for `requestBody`;
([bug report](https://github.com/Neoteroi/essentials-openapi/issues/21)).
- Fixes support for code fences (disables by default `autoescape`, since the
source of OpenAPI Specification files is supposed to be trusted anyway.
Those who still wants to have `autoescape` enabled with `Jinja` can do so
setting an environment variable: `SELECT_AUTOESCAPE=1`.
([bug report](https://github.com/Neoteroi/essentials-openapi/issues/24)).

- Fixes [22](https://github.com/Neoteroi/essentials-openapi/issues/22)

- Fixes [18](https://github.com/Neoteroi/essentials-openapi/issues/18)
- Workflow maintenance