src/sage/combinat/cluster_algebra_quiver/quiver_mutation_type.py: Use lazy_import

Author: Matthias Koeppe

src/sage/combinat/cluster_algebra_quiver/quiver_mutation_type.py

@@ -23,13 +23,15 @@
 from copy import copy
 from sage.structure.unique_representation import UniqueRepresentation
 from sage.misc.cachefunc import cached_method
+from sage.misc.lazy_import import lazy_import
 from sage.rings.integer_ring import ZZ
 from sage.rings.infinity import infinity
 from sage.graphs.digraph import DiGraph
 from sage.graphs.graph import Graph
 from sage.arith.misc import binomial, euler_phi
 from sage.misc.misc_c import prod
-from sage.matrix.constructor import matrix
+
+lazy_import('sage.matrix.constructor', 'matrix')
 
 
 class QuiverMutationTypeFactory(SageObject):