go get github.com/ProtonMail/go-crypto

This module is backwards compatible with x/crypto/openpgp, so you can simply replace all imports of golang.org/x/crypto/openpgp with github.com/ProtonMail/go-crypto/openpgp.

A partial list of changes is here: ProtonMail#21 (comment).

For the more extended API for reading and writing OpenPGP messages use github.com/ProtonMail/go-crypto/openpgp/v2, but it is not fully backwards compatible with golang.org/x/crypto/openpgp.