Release v3.1.0-proton.1

This release is v3.1.0 with support for the following non-standardized features:

• Presistent symmetric keys draft-ietf-openpgp-persistent-symmetric-keys-00
• Automatic forwarding draft-wussler-openpgp-forwarding-00
• Post-quantum algorithms draft-ietf-openpgp-pqc
• Proton profiles

Patches v3.1.0-proton:

• Update go-crypto to v3.1.0-proton.1 to fix forwarding in the go-crypto/v2 API.