[python] Big UInt gets generated as negative int literal.

```haxe
class Main {
static function main() {
var u:UInt = 0x80000000;
Sys.println(u);
}
}
```

Yields:

```python
class Main:
_hx_class_name = "Main"
__slots__ = ()
_hx_statics = ["main"]

@staticmethod
def main():
u = -2147483648
_hx_str = Std.string(u)
python_Lib.printString((("" + ("null" if _hx_str is None else _hx_str)) + HxOverrides.stringOrNull(python_Lib.lineEnd)))
```

Start by reproducing the Haxe example from the issue and inspecting the generated Python output. Trace the Python-target code that handles the UInt literal; done means the generated literal preserves the intended unsigned value and the example prints the correct result.

Written by the indexing model from the issue text.