v0.3.0

What's Changed

• add makefile and update go.mod to Go1.21 by @aead in #28
• add support for passwordless private keys by @aead in #29
• improve docs and implementation of Signature, PrivateKey by @aead in #30
• add -C flag for changing private key password by @aead in #31
• adjust private key tests by @aead in #32
• update version to v0.3.0 by @aead in #33

Full Changelog: v0.2.1...v0.3.0