Currently pre-signed URLs generated using the AWS v4 signature type are not supported. Unfortunately, v4 signatures are the default in most places, so this can cause some issues. Luckily, most clients allow you to override this.

Using boto3, you can configure your session to use the v2 signature type with:

import boto3
from botocore.client import Config

session = boto3.session.Session()

client = session.client('s3',
                        region_name='nyc3',
                        endpoint_url='https://nyc3.digitaloceanspaces.com',
                        aws_access_key_id='MYACCESSKEY',
                        aws_secret_access_key='MYSECRETKEY',
                        config=Config(signature_version='s3'))

(Passing s3 as the value for signature_version will force v3 signature. While v4 is the defualt, you can explicitly use it by passing s3v4.)

Then you can generate a functioning pre-signed url using:

url = client.generate_presigned_url(ClientMethod='get_object', 
                                    Params={'Bucket': 'my-bucket',
                                            'Key': 'my-object'},
                                    ExpiresIn=300)

Note the ExpiresIn argument. By default, pre-signed URLs will expire in an hour (3600 seconds). This example sets it to expire in 5 minutes. See the boto3 docs for more info.