More cleanup

skimage/graph/_mcp.pyx

@@ -36,7 +36,7 @@ THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 import cython
 import numpy as np
 import heap
-import warnings
+from .._shared.utils import warn
 
 cimport numpy as cnp
 cimport heap
@@ -304,7 +304,8 @@ cdef class MCP:
             self.flat_costs = costs.astype(FLOAT_D, copy=False).ravel('F')
         except TypeError:
             self.flat_costs = costs.astype(FLOAT_D).flatten('F')
-            warnings.warn('Upgrading NumPy should decrease memory usage and increase speed.', Warning, stacklevel=2)
+            warn('Upgrading NumPy should decrease memory usage and increase'
+                 ' speed.')
         size = self.flat_costs.shape[0]
         self.flat_cumulative_costs = np.empty(size, dtype=FLOAT_D)
         self.dim = len(costs.shape)