gh-36896: `src/sage/interfaces/{gap,kenzo,scilab}`: fix doctests warnings
    
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()
```


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URL: #36896
Reported by: David Coudert
Reviewer(s): Matthias Köppe

Author: Release Manager

src/sage/interfaces/gap.py

@@ -1509,20 +1509,21 @@ def gap_reset_workspace(max_workspace_size=None, verbose=False):
     We temporarily need to change the worksheet filename, and to set
     ``first_try=True`` to ensure that the new workspace is created::
 
+        sage: # long time
         sage: ORIGINAL_WORKSPACE = sage.interfaces.gap.WORKSPACE
         sage: saved_first_try = sage.interfaces.gap.first_try
         sage: sage.interfaces.gap.first_try = True
         sage: sage.interfaces.gap.WORKSPACE = tmp_filename()
         sage: from multiprocessing import Process
         sage: import time
-        sage: gap = Gap()  # long time (reset GAP session)
+        sage: gap = Gap()  # reset GAP session
         sage: P = [Process(target=gap, args=("14242",)) for i in range(4)]
-        sage: for p in P:  # long time, indirect doctest
+        sage: for p in P:  # indirect doctest
         ....:     p.start()
         ....:     time.sleep(float(0.2))
-        sage: for p in P:  # long time
+        sage: for p in P:
         ....:     p.join()
-        sage: os.unlink(sage.interfaces.gap.WORKSPACE)  # long time
+        sage: os.unlink(sage.interfaces.gap.WORKSPACE)
         sage: sage.interfaces.gap.WORKSPACE = ORIGINAL_WORKSPACE
         sage: sage.interfaces.gap.first_try = saved_first_try
     """