Move API version to `base_url`

[kylemclaren]

i.e. the base_url should be https://api.replicate.com/v1 and v1 should be dropped in all client request paths. This is in line with the the JavaScript client, but also allows using a custom cog endpoint with the Replicate client (at least for prediction endpoints).

https://github.com/replicate/replicate-python/blob/d7ac9425299a3b8878ec56b089d61f8c47cd39ec/replicate/client.py#L348-L350