wrong error message when deleting a vertex from a static sparse graph

[dcoudert]

Steps To Reproduce

sage: G = Graph([('a', 1), (1, 2), (2, 3)], immutable=True)
sage: type(G._backend)
<class 'sage.graphs.base.static_sparse_backend.StaticSparseBackend'>

Expected Behavior

sage: G.delete_vertex(1)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
...
ValueError: graph is immutable; please change a copy instead (use function copy())
sage: G.delete_vertex('a')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
...
ValueError: graph is immutable; please change a copy instead (use function copy())

Actual Behavior

sage: G.delete_vertex(1)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
...
ValueError: graph is immutable; please change a copy instead (use function copy())
sage: G.delete_vertex('a')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
...
TypeError: an integer is required

Additional Information

No response

Environment

• OS: all
• Sage Version: 10.6.beta2

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide