sagemathgh-38592: Add is_integral method to algebraic numbers

    
For symmetry with existing methods e.g. `QQ(1).is_integral` or such
method on elements of number fields.

Implementation pretty much copied from `NumberFieldElement`.

### 📝 Checklist

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preview.
    
URL: sagemath#38592
Reported by: user202729
Reviewer(s): Kwankyu Lee, user202729

src/sage/rings/qqbar.py

@@ -4485,6 +4485,26 @@ def as_number_field_element(self, minimal=False, embedded=False, prec=53):
         """
         return number_field_elements_from_algebraics(self, minimal=minimal, embedded=embedded, prec=prec)
 
+    def is_integral(self):
+        r"""
+        Check if this number is an algebraic integer.
+
+        EXAMPLES::
+
+            sage: QQbar(sqrt(-23)).is_integral()
+            True
+            sage: AA(sqrt(23/2)).is_integral()
+            False
+
+        TESTS:
+
+        Method should return the same value as :meth:`NumberFieldElement.is_integral`::
+
+             sage: for a in [QQbar(2^(1/3)), AA(2^(1/3)), QQbar(sqrt(1/2)), AA(1/2), AA(2), QQbar(1/2)]:
+             ....:    assert a.as_number_field_element()[1].is_integral() == a.is_integral()
+        """
+        return all(a in ZZ for a in self.minpoly())
+
     def exactify(self):
         """
         Compute an exact representation for this number.