Remove upper bound from Python version specifier

[sisp]

Capping the Python version (e.g. <4) is a bad practice.

• fix: Remove '<4' from python_requires in packaging guide pypa/packaging.python.org#850
• https://iscinumpy.dev/post/bound-version-constraints/#pinning-the-python-version-is-special
• https://discuss.python.org/t/requires-python-upper-limits/12663

Hence, I've removed the upper bound, i.e. I've replaced the version specifier ^ by >=.

[ParthSareen]

If we end up keeping poetry will get this in :)

[sisp]

Any reason not to merge this, even if you consider migrating away from Poetry at some point?