Python compiler: ensure_fixed_contents uses struct not literal
I was looking at Bitmap example, Python gen in particular and noticed that Magic field emits a line:
http://formats.kaitai.io/bmp/python.html
```py
self.magic = self._io.ensure_fixed_contents(struct.pack('2b', 66, 77))
```

Struct.pack is not particularly speedy. It would be good to have a literal instead. I am not sure if that should be done in preprocessing or in python-specific compiler. I tracked the emit to this line and got lost:
https://github.com/kaitai-io/kaitai_struct_compiler/blob/349015054f016f345f95d0a8adde344d6eaf1512/shared/src/main/scala/io/kaitai/struct/languages/PythonCompiler.scala#L167
```scala
  override def attrFixedContentsParse(attrName: Identifier, contents: String): Unit =
    out.puts(s"${privateMemberName(attrName)} = self._io.ensure_fixed_contents($contents)")
```

This is a low priority upgrade, because Magic is sporadically used and its already fast enough. Its just that it could be little faster. 
