sagemathgh-39757: Ensure _charpoly_df is interruptible
    
As in the title.

Note that `sig_check()` is only called every `O(t)` ring operations, but
each of them should be `O(1)`, otherwise the implementation of the ring
operation itself ought to check it… I think.

Or do you think it's better to check every iteration instead?

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] 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

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#39757
Reported by: user202729
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/matrix/matrix2.pyx

@@ -3291,6 +3291,16 @@ cdef class Matrix(Matrix1):
             sage: A._charpoly_df()
             x^3 + 8*x^2 + 10*x + 1
 
+        .. NOTE::
+
+            The key feature of this implementation is that it is division-free.
+            This means that it can be used as a generic implementation for any
+            ring (commutative and with multiplicative identity).  The algorithm
+            is described in full detail as Algorithm 3.1 in [Sei2002]_.
+
+            Note that there is a missing minus sign in front of the last term in
+            the penultimate line of Algorithm 3.1.
+
         TESTS::
 
             sage: A = matrix(ZZ, 0, 0)
@@ -3309,15 +3319,11 @@ cdef class Matrix(Matrix1):
             sage: matrix(4, 4, lambda i, j: R.an_element())._charpoly_df()              # needs sage.combinat
             B[1]*x^4 - 4*B[u]*x^3
 
-        .. NOTE::
+        Test that the function is interruptible::
 
-            The key feature of this implementation is that it is division-free.
-            This means that it can be used as a generic implementation for any
-            ring (commutative and with multiplicative identity).  The algorithm
-            is described in full detail as Algorithm 3.1 in [Sei2002]_.
-
-            Note that there is a missing minus sign in front of the last term in
-            the penultimate line of Algorithm 3.1.
+            sage: m = matrix.random(RR, 128)
+            sage: from sage.doctest.util import ensure_interruptible_after
+            sage: with ensure_interruptible_after(1): m.charpoly()
         """
 
         # Validate assertions
@@ -3374,6 +3380,7 @@ cdef class Matrix(Matrix1):
                     for j in range(t+1):
                         s = s + M.get_unsafe(i, j) * a.get_unsafe(p-1, j)
                     a.set_unsafe(p, i, s)
+                    sig_check()
 
                 # Set A[p, t] to be the (t)th entry in a[p, t]
                 A[p] = a.get_unsafe(p, t)