src/sage/interfaces/{gap,kenzo,scilab}: fix doctests warnings

[dcoudert]

Fix warnings like

File "src/sage/interfaces/gap.py", line 1509, in sage.interfaces.gap.gap_reset_workspace
Warning: Variable 'gap' referenced here was set only in doctest marked '# long time'
    P = [Process(target=gap, args=("14242",)) for i in range(4)]

File "src/sage/interfaces/kenzo.py", line 134, in sage.interfaces.kenzo.Sphere
Warning: Consider using a block-scoped tag by inserting the line 'sage: # optional - kenzo' just before this line to avoid repeating the tag 4 times
    from sage.interfaces.kenzo import Sphere # optional - kenzo

File "src/sage/interfaces/scilab.py", line 252, in sage.interfaces.scilab.Scilab.set_seed
Warning: Consider using a block-scoped tag by inserting the line 'sage: # optional - scilab' just before this line to avoid repeating the tag 4 times
    from sage.interfaces.scilab import Scilab # optional - scilab

We also use more block scope tags.

However, we let one warning in src/sage/interfaces/gp.py. This is a doctest with # 32-bit and # 64-bit tags.

File "src/sage/interfaces/gp.py", line 799, in sage.interfaces.gp.Gp.new_with_bits_prec
Warning: Variable 'pi_def' referenced here was set only in doctest marked '# needs sage.libs.pari sage.symbolic'
    pi_def.precision()

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 78ac95b; changes) is ready! 🎉

[mkoeppe]

LGTM

[mkoeppe]

we let one warning in src/sage/interfaces/gp.py. This is a doctest with # 32-bit and # 64-bit tags.

Yes, that's #36099

[dcoudert]

Thank you.