Encrypting credentials and reusing them securely
Do the encryption and decryption on the second server (encryption server).
Pass the password to the encryption server along with an id for encryption and it returns the encrypted password to store in the DB.
When the password is needed pass the encrypted password to the encryption server for decryption.
Have the encryption server monitor request activity, if an unusual number of requests are received sound an alarm and in extreme cases stop processing requests.
Make the second server very secure. No Internet access, minimal access accounts, 2-factor authentication.
The encryption server becomes a poor-man's HSM (Hardware Encryption Module).
I'm currently building a website where the users would enter their credentials for another web service that I'm going to scrape to get their data.
I want to make sure that when I save their credentials in my database, I'm using the best encryption possible and the best architecture to ensure the highest level of security.
The first idea that I had in mind was to encrypt the data using an RSA pub key (PBKDF2, PKCS1_OAEP, AES 256bit... ???) and then allowing my scrapping script to use the private key to decrypt the credentials and use them.

But if my server is hacked, the hacker would have access to both the database and the private key, since it will be kept on my server that runs the scrapping script and hosts the DB. Is there an architecture pattern that solves this ?
I've read that that there should be a mix of hashing and encryption to enable maximum security but hashing is uni directional and it doesn't fit my use case since I will have to reuse the credentials. If you can advise me with the best encryption cypher/pattern you know it could be awesome.

I'm coding in python and I believe PyCrypto is the go-to library for encryption. (Sorry I have very little knowledge about cryptography so I might be confusing technologies)