Boto3 Store is untested code

[siboehm]

In SimpleKV the Boto3 store was merged while the CI had been down for months: mbr/simplekv#106

So there was never a successful test-run on the CI. I spent an hour on it but couldn't figure out how to get it to connect to a Minio instance without bigger changes to the code. This is due to the client being created implicitly (contrary to explicitly like in the Boto store). I couldn't find a way to point the client to a local endpoint without modifying the global ~/.aws/config file.

The boto store still works, but the boto library hasn't seen any updates since 2018, so we should move to boto3 at some point definitely.

[simonbohnen]

The tests for the Boto3Store are running, see e.g. here for failure vs here for passing tests.

[simonbohnen]

@xhochy could you close this?