gh-36553: Add a hash function for the class TernaryQF.
    
PR from #16956 (rebased from https://github.com/sagemath/sagetrac-
mirror/commit/091cbbdf6eed2c8bccca407addd07ff08e429c17)

Author: Gustavo Rama

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
    
URL: #36553
Reported by: Gonzalo Tornaría
Reviewer(s):

Author: Release Manager

src/sage/quadratic_forms/ternary_qf.py

@@ -116,6 +116,20 @@ def coefficients(self):
         """
         return self._a, self._b, self._c, self._r, self._s, self._t
 
+    def __hash__(self):
+        """
+        Returns a hash for self.
+
+        EXAMPLES::
+
+            sage: Q = TernaryQF([1, 2, 3, 4, 5, 6])
+            sage: Q.__hash__()
+            5881802312257552497  # 64-bit
+            1770036893           # 32-bit
+        """
+
+        return hash(self.coefficients())
+
     def coefficient(self, n):
         r"""
         Return the `n`-th coefficient of the ternary quadratic form.