File backend hashlib.md5 not supported in FIPS mode

[wonkybream]

I have a quite specific environment and noticed one issue with it.

My environment is in OpenSSL FIPS 140-2 mode and using md5 is not supported.

Following line raises an error ValueError: [digital envelope routines: FIPS_DIGESTINIT] disabled for fips.

celery-once/celery_once/backends/file.py

Line 20 in 4f68b6b

key_hash = hashlib.md5(six.b(key)).hexdigest()

I'm suggesting to change that to hashlib.sha1() which is supported in even if FIPS mode is enabled in cryptographic library.

If proposed change is allowed I'm more than happy to do the pull request.

With best regards,
Wonky