docs: def fingerprint_from_file(filename):
      """Extract a fingerprint from a GPG public key file"""
      cmd = flatten([gnupg_bin(), gnupg_home(), filename])
      outp = stderr_output(cmd).split('\n')
      if not outp[0].startswith('pub'):
          raise CryptoritoError('probably an invalid gpg key')
  
      return outp[1].strip()