If `mlab` is imported, `matplotlib` stops working in a multiprocessing environment

Consider the following MWE:

```python
#!/usr/bin/env python3

import concurrent.futures

from mayavi import mlab
import matplotlib.pyplot as plt

list_ = [1]

def function(_):
plt.figure()

with concurrent.futures.ProcessPoolExecutor() as ex:
futures = {ex.submit(function, _): _ for _ in list_}
```

When I run this, it hangs forever. But if I comment out the import of `mlab`, it does not hang.

This is with `python` 3.8.7, `matplotlib` 3.3.4 and `mayavi` 4.7.2.

I am happy to provide additional information.