Accept memory resource (and stream) in copy

Changes `copy` to allow a memory resource to be specified (using the
default device memory resource if unspecified). Also accepts a `stream`
argument (similar to other functions).

python/rmm/_lib/device_buffer.pyx

@@ -146,7 +146,9 @@ cdef class DeviceBuffer:
         }
         return intf
 
-    def copy(self):
+    def copy(self, *,
+             Stream stream=DEFAULT_STREAM,
+             DeviceMemoryResource mr=None):
         """Returns a copy of DeviceBuffer.
 
         Returns
@@ -165,9 +167,9 @@ cdef class DeviceBuffer:
         >>> assert db is not db_copy
         >>> assert db.ptr != db_copy.ptr
         """
-        ret = DeviceBuffer(ptr=self.ptr, size=self.size, stream=self.stream)
-        ret.mr = self.mr
-        return ret
+        return DeviceBuffer(
+            ptr=self.ptr, size=self.size, stream=stream, mr=mr
+        )
 
     def __copy__(self):
         return self.copy()