Remove upper constraint on Python < 4

[edgarrmondragon]

It causes problems for downstream dependents, even indirectly, specially
for Poetry. Most recently:

Because my-package depends on rsa (4.7) which requires Python >=3.5, <4, version solving failed.

  • Check your dependencies Python requirement: The Python requirement can be specified via the `python` or `markers` properties

    For rsa, a possible solution would be to set the `python` property to ">=3.8,<4"

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

From the comments in the Discourse by Core developers and SC members, there probably won't be a Python 4 so I think it's fine to remove this constraint.