Use a.item() instead of deprecated np.asscalar(a)

np.asscalar() has been deprecated in numpy 1.16:

https://github.com/numpy/numpy/blob/master/doc/release/1.16.0-notes.rst#new-deprecations

src/_pytest/python_api.py

@@ -150,10 +150,10 @@ class ApproxNumpy(ApproxBase):
 
         if np.isscalar(actual):
             for i in np.ndindex(self.expected.shape):
-                yield actual, np.asscalar(self.expected[i])
+                yield actual, self.expected[i].item()
         else:
             for i in np.ndindex(self.expected.shape):
-                yield np.asscalar(actual[i]), np.asscalar(self.expected[i])
+                yield actual[i].item(), self.expected[i].item()
 
 
 class ApproxMapping(ApproxBase):