Change warning category for force_type to DeprecationWarning

Closes #156

mpmath/matrices/matrices.py

@@ -285,7 +285,8 @@ class _matrix:
                 " properly anyway. If you want to force floating-point or"
                 " interval computations, use the respective methods from `fp`"
                 " or `mp` instead, e.g., `fp.matrix()` or `iv.matrix()`."
-                " If you want to truncate values to integer, use .apply(int) instead.")
+                " If you want to truncate values to integer, use .apply(int) instead.",
+                DeprecationWarning)
         if isinstance(args[0], (list, tuple)):
             if isinstance(args[0][0], (list, tuple)):
                 # interpret nested list as matrix

mpmath/tests/test_matrices.py

@@ -251,3 +251,7 @@ def test_interval_matrix_mult_bug():
     assert mp.mpf('1.00000000000001998401444325291756783368705994138804689654') in C[0, 0]
     # the following caused an error before the bug was fixed
     assert iv.matrix(mp.eye(2)) * (iv.ones(2) + mpi(1, 2)) == iv.matrix([[mpi(2, 3), mpi(2, 3)], [mpi(2, 3), mpi(2, 3)]])
+
+def test_issue_156():
+    with pytest.deprecated_call():
+        matrix([[1, 2], [3, 4]], force_type=float)