gh-39572: Change some instances of "gens" method to return tuples

    
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Contributes to #34120

As noted in the referenced issue. most instances of the "gens" method
return tuples, so the few instances of the method which return lists
instead should be modified to return tuples. This pull request handles
the following bullet points from the issue:
- `SymmetricReductionStrategy` in
`src/sage/rings/polynomial/symmetric_reduction.pyx`
- `EllipticCurve_number_field` in
`src/sage/schemes/elliptic_curves/ell_number_field.py`
- `SpecialCubicQuotientRing` in
`src/sage/schemes/hyperelliptic_curves/monsky_washnitzer.py`

In each case the gens method from the mentioned class has been modified
to return a tuple instead of a list and doctests have been updated
appropriately.

### 📝 Checklist

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- [X] I have linked a relevant issue or discussion.
- [X] I have created tests covering the changes.
- [X] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: #39572
Reported by: Caleb Van't Land
Reviewer(s): Vincent Macri

src/sage/rings/polynomial/symmetric_reduction.pyx

@@ -254,9 +254,9 @@ cdef class SymmetricReductionStrategy:
         return richcmp((left._parent, left._lm, left._tail),
                        (right._parent, right._lm, right._tail), op)
 
-    def gens(self) -> list:
+    def gens(self) -> tuple:
         """
-        Return the list of Infinite Polynomials modulo which ``self`` reduces.
+        Return the tuple of Infinite Polynomials modulo which ``self`` reduces.
 
         EXAMPLES::
 
@@ -269,9 +269,9 @@ cdef class SymmetricReductionStrategy:
                 y_2*y_1^2,
                 y_2^2*y_1
             sage: S.gens()
-            [y_2*y_1^2, y_2^2*y_1]
+            (y_2*y_1^2, y_2^2*y_1)
         """
-        return self._lm
+        return tuple(self._lm)
 
     def setgens(self, L):
         """

src/sage/schemes/elliptic_curves/ell_number_field.py

@@ -158,7 +158,7 @@ def base_extend(self, R):
             [(52 : 111 : 1)]
             sage: EK = E.base_extend(K)
             sage: EK.gens()
-            [(52 : 111 : 1)]
+            ((52 : 111 : 1),)
         """
         E = super().base_extend(R)
         if isinstance(E, EllipticCurve_number_field):
@@ -2328,21 +2328,21 @@ def gens(self, **kwds):
             sage: K.<a> = NumberField(x^2 + 23, 'a')
             sage: E = EllipticCurve(K,[0,0,0,101,0])
             sage: E.gens()
-            [(23831509/8669448*a - 2867471/8669448 : 76507317707/18049790736*a - 424166479633/18049790736 : 1),
+            ((23831509/8669448*a - 2867471/8669448 : 76507317707/18049790736*a - 424166479633/18049790736 : 1),
              (-2031032029/969232392*a + 58813561/969232392 : -15575984630401/21336681877488*a + 451041199309/21336681877488 : 1),
-             (-186948623/4656964 : 549438861195/10049728312*a : 1)]
+             (-186948623/4656964 : 549438861195/10049728312*a : 1))
 
         It can happen that no points are found if the height bounds
         used in the search are too small (see :issue:`10745`)::
 
             sage: K.<t> = NumberField(x^4 + x^2 - 7)
             sage: E = EllipticCurve(K, [1, 0, 5*t^2 + 16, 0, 0])
             sage: E.gens(lim1=1, lim3=1)
-            []
+            ()
             sage: E.rank()
             1
             sage: gg=E.gens(lim3=13); gg  # long time (about 4s)
-            [(... : 1)]
+            ((... : 1),)
 
         Check that the point found has infinite order, and that it is on the curve::
 
@@ -2356,7 +2356,7 @@ def gens(self, **kwds):
             sage: K.<t> = NumberField(x^2 - 17)
             sage: E = EllipticCurve(K, [-4, 0])
             sage: E.gens()
-            [(-1/2*t + 1/2 : -1/2*t + 1/2 : 1), (-t + 3 : -2*t + 10 : 1)]
+            ((-1/2*t + 1/2 : -1/2*t + 1/2 : 1), (-t + 3 : -2*t + 10 : 1))
             sage: E.rank()
             2
 
@@ -2368,7 +2368,7 @@ def gens(self, **kwds):
             sage: EK.rank()
             0
             sage: EK.gens()
-            []
+            ()
 
         IMPLEMENTATION:
 
@@ -2378,10 +2378,10 @@ def gens(self, **kwds):
         PARI/GP scripts from http://www.math.unicaen.fr/~simon/.
         """
         try:
-            return self.gens_quadratic(**kwds)
+            return tuple(self.gens_quadratic(**kwds))
         except ValueError:
             self.simon_two_descent(**kwds)
-            return self._known_points
+            return tuple(self._known_points)
 
     def period_lattice(self, embedding):
         r"""

src/sage/schemes/hyperelliptic_curves/monsky_washnitzer.py

@@ -572,7 +572,7 @@ def poly_ring(self):
 
     def gens(self):
         """
-        Return a list [x, T] where x and T are the generators of the ring
+        Return a tuple (x, T) where x and T are the generators of the ring
         (as element *of this ring*).
 
         .. NOTE::