gh-36588: Replace relative imports by absolute ones in rings
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As preparation for compiling with meson, we replace relative imports by
absolute ones.
Relative imports are not used consistently in the codebase and result in
issues for doctesting with pytest (which admittedly is a limitation of
pytest). We normalize the relative imports in `sage.rings` to be
absolute imports. Small code-style improvements along the way (mainly to
nicely order the imports)
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URL: https://github.com/sagemath/sage/pull/36588
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik
