Use looser version constraints in `pyproject.toml`

`pyproject.toml` currently specifies exact version constraints, like your `*-requirements.txt` files.

This is a problem for software distributions which package schema-salad, as then any update of any dependency breaks the build.
I encountered this when [updating `mypy` in `nixpkgs`] and had to patch the released version of schema-salad (applying the change from the update bot)

Many projects have a documented version policy; in the case of `mypy`, for instance, accepting `1.10.*` (rather than `1.10.0` only) would make sense, as those would only match bugfix versions in mypy's 1.10 branch.

[updating `mypy` in `nixpkgs`]: https://github.com/NixOS/nixpkgs/pull/324867