Make code compatible with Ruby 2.7

Not including specs etc.

lib/nostr/errors/invalid_key_type_error.rb

@@ -13,6 +13,8 @@ module Nostr
     #
     # @param [String] key_kind The kind of key that is invalid (public or private)
     #
-    def initialize(key_kind) = super("Invalid #{key_kind} key type")
+    def initialize(key_kind)
+      super("Invalid #{key_kind} key type")
+    end
   end
 end