build: Remove upper bound on requires-python

[matthewfeickert]

Remove upper bound on requires-python as advocated for in Henry Schreiner's blog post "Should You Use Upper Bound Version Constraints?".

• c.f. https://iscinumpy.dev/post/bound-version-constraints/#pinning-the-python-version-is-special

c.f. https://github.com/poliastro/poliastro/pull/1588 for reference

[matthewfeickert]

@jmschrei this is ready for review.

[jmschrei]

Thanks!