Remove upper Python version constraint

[RafaelWO]

This PR removes the upper Python version constraint (<4.0.0) in pyproject.toml.

The idea behind this change is that users of autodoc_pydantic can have issues installing it in a downstream package if their package does not have such a constraint (see command output below).

Click for command output

$ poetry add "autodoc_pydantic[erdantic]"
Using version ^2.2.0 for autodoc-pydantic

Updating dependencies
Resolving dependencies... (0.1s)

The current project's supported Python range (>=3.9) is not compatible with some of the required packages Python requirement:
  - autodoc-pydantic requires Python <4.0.0,>=3.8.1, so it will not be satisfied for Python >=4.0.0

Because no versions of autodoc-pydantic match >2.2.0,<3.0.0
 and autodoc-pydantic[erdantic] (2.2.0) requires Python <4.0.0,>=3.8.1, autodoc-pydantic is forbidden.
So, because test-package-template depends on autodoc-pydantic[erdantic] (^2.2.0), version solving failed.

  • Check your dependencies Python requirement: The Python requirement can be specified via the `python` or `markers` properties
    
    For autodoc-pydantic, a possible solution would be to set the `python` property to ">=3.9,<4.0.0"

    https://python-poetry.org/docs/dependency-specification/#python-restricted-dependencies,
    https://python-poetry.org/docs/dependency-specification/#using-environment-markers

Furthermore, other prominent packages also do not have this upper constrain, e.g. FastAPI, rich, or transformers, so it should be fine 🙂