gh-36589: Replace relative imports by absolute ones in a few packages
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As preparation for compiling with meson, we replace relative imports by
absolute ones.
Moreover, 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 a few
smaller packages 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/36589
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik
