Migrate S3FileAdmin from boto to boto3

[tgross35]

Boto is no longer maintained, and has been replaced by boto3. This is described on the two project pages: https://github.com/boto/boto and https://github.com/boto/boto3. However, flask-admin still relies on the older boto:

Could not import boto. You can install boto by using pip install boto

The migration is fairly straightforward

https://boto3.amazonaws.com/v1/documentation/api/latest/guide/migration.html