gh-35957: `sage.rings.function_field`: Update `# needs`
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- Many `# optional` are removed because small prime finite fields no
longer need to be marked `# optional/needs sage.rings.finite_rings`
since #35847
- Other `# optional` are replaced by codeblock-scoped `# needs` tags
- Remaining `# optional` with standard features are replaced by `#
needs`
- Some import fixes needed for separate testing of **sagemath-graphs**
from #35095
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This is:
- done with the help of `sage -fixdoctests` from #35749
- Part of: #29705
- Cherry-picked from: #35095
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- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35957
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Matthias Köppe
