Python/Construct: Use unsigned char representation when converting …

…to bytes using `.as<bytes>`

That means that converted array expected to be an array of integers in the range [0, 255].

Perl and Ruby also uses unsigned representation ('C*')

jvm/src/test/scala/io/kaitai/struct/translators/TranslatorSpec.scala

@@ -675,7 +675,7 @@ class TranslatorSpec extends AnyFunSpec {
         LuaCompiler -> "???",
         PerlCompiler -> "pack('C*', ((0 + 1), 5))",
         PHPCompiler -> "pack('C*', (0 + 1), 5)",
-        PythonCompiler -> "struct.pack('2b', (0 + 1), 5)",
+        PythonCompiler -> "struct.pack('2B', (0 + 1), 5)",
         RubyCompiler -> "[(0 + 1), 5].pack('C*')"
       ))
 

shared/src/main/scala/io/kaitai/struct/translators/PythonTranslator.scala

@@ -40,7 +40,8 @@ class PythonTranslator(provider: TypeProvider, importList: ImportList) extends B
     "b\"" + Utils.hexEscapeByteArray(arr) + "\""
   override def doByteArrayNonLiteral(elts: Seq[Ast.expr]): String = {
     importList.add("import struct")
-    s"struct.pack('${elts.length}b', ${elts.map(translate).mkString(", ")})"
+    // Use `unsigned char` representation (see https://docs.python.org/3/library/struct.html#struct-format-strings)
+    s"struct.pack('${elts.length}B', ${elts.map(translate).mkString(", ")})"
   }
 
   override def doLocalName(s: String) = {