MAINT: Update min supported versions and add 3.15 to CI

[derb12]

Description

Bumps the minimum supported python version to 3.11 and updated min supported dependencies. Decided to not follow SPEC 0 as mentioned in PR #94 since it'd end up dropping 4 python versions since the last release. This should hopefully be the last min version bump before v1.3 is released.

Also adds python 3.15 to CI; will update the free-threaded job from 3.14t to 3.15t once Numba supports it.

Type of Pull Request

• Bug Fix
• New Feature
• Miscellaneous Changes (refactor, code improvements, etc.)
• Documentation or Example Programs

Pull Request Checklist

• New code and/or documentation is valid for use with the BSD 3-clause license.
• [N/A] New code is fully documented with docstrings that follow Numpy style,
  if applicable.
• [N/A] New code follows PEP 8 standards as closely as possible, if applicable.
• Added/updated tests and ensured they pass locally, if applicable.
• [N/A] Verified that documentation builds locally, if applicable.

[derb12]

Whoops, forgot to update the nightly job to also use 3.15; will do after merging since it's not worth another CI run and it passes locally using act.