Multiprocessing with FileLock fails in python 3.9
Closing this one because it must be used this way actually:
```python
def main():
    with FileLock("tmp.lock"):
        with Pool(2) as pool:
            pool.map(run, range(2))

if __name__ == "__main__":
    main()
```
On python 3.9, this code hangs:
```python
from multiprocessing import Pool
from filelock import FileLock


def run(i):
    print(f"got the lock in multi process [{i}]")


with FileLock("tmp.lock"):
    with Pool(2) as pool:
        pool.map(run, range(2))

```

This is because the subprocesses try to acquire the lock from the main process for some reason. This is not the case in older versions of python.

This can cause many issues in python 3.9. In particular, we use multiprocessing to fetch data files when you load a dataset (as long as there are >16 data files). Therefore `imagefolder` hangs, and I expect any dataset that needs to download >16 files to hang as well.

Let's see if we can fix this and have a CI that runs on 3.9.

cc @mariosasko @julien-c 