gh-37306: Fix random test failures in `number_field_element_quadratic`
    
Fix #37296.

Add `warnings.filterwarnings` to ignore `DeprecationWarning` in the
failing doctest, since it is an unrelated issue and does not affect
correctness of the test.

To be more specific, the warning only appears when the rounding error
will change, i.e. on $n + \frac{1}{2}$ (I believe). Here are some runs:

```python
sage: a = QQ.random_element(3, 3); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
<ipython-input-12-179149e36f01>:1: DeprecationWarning: the default
rounding for rationals, currently `away`, will be changed to `even`.
See #35473 for details.
  a = QQ.random_element(Integer(3), Integer(3)); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
<ipython-input-14-179149e36f01>:1: DeprecationWarning: the default
rounding for rationals, currently `away`, will be changed to `even`.
See #35473 for details.
  a = QQ.random_element(Integer(3), Integer(3)); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
sage: a = QQ.random_element(3, 3); _ = round(a)
```
    
URL: #37306
Reported by: grhkm21
Reviewer(s): Sebastian A. Spindler

Author: Release Manager

src/sage/rings/number_field/number_field_element_quadratic.pyx

@@ -2387,6 +2387,8 @@ cdef class NumberFieldElement_quadratic(NumberFieldElement_absolute):
 
         TESTS::
 
+            sage: import warnings
+            sage: warnings.filterwarnings("ignore", category=DeprecationWarning)
             sage: K2.<sqrt2> = QuadraticField(2)
             sage: K3.<sqrt3> = QuadraticField(3)
             sage: K5.<sqrt5> = QuadraticField(5)
@@ -2399,8 +2401,6 @@ cdef class NumberFieldElement_quadratic(NumberFieldElement_absolute):
             ....:    assert round(a+b*sqrt(2.)) == round(a+b*sqrt2), (a, b)
             ....:    assert round(a+b*sqrt(3.)) == round(a+b*sqrt3), (a, b)
             ....:    assert round(a+b*sqrt(5.)) == round(a+b*sqrt5), (a, b)
-            doctest...: DeprecationWarning: the default rounding for rationals, currently `away`, will be changed to `even`.
-            See https://github.com/sagemath/sage/issues/35473 for details.
         """
         n = self.floor()
         test = 2 * (self - n).abs()