concatenate of DOK fails

[dschwoerer]

Describe the bug
concatentating a DOK fails

To Reproduce

import sparse
import numpy as np

x = np.eye(4)
x = sparse.DOK.from_numpy(x)
sparse.concatenate([x])

Expected behavior
Similar result to COO version, i.e. combined array

System

• OS and version: Fedora 33
• sparse version (current master)
• NumPy version (1.19.4)
• Numba version (0.53.1)