[PyQt] BUG: example lightmaps.py needs to be updated for PyQt v5.5+

[Anthony Fok]

Hello,

Running examples/network/lightmaps.py yields the following error in
PyQt v5.5+ due to the removal of automatic (latin_1) str to QByteArray
conversion:

======================================================
$ python3 examples/network/lightmaps.py
Traceback (most recent call last):
  File "examples/network/lightmaps.py", line 280, in resizeEvent
    self._normalMap.invalidate()
  File "examples/network/lightmaps.py", line 168, in invalidate
    self.download()
  File "examples/network/lightmaps.py", line 222, in download
    request.setRawHeader('User-Agent', 'Nokia (PyQt) Graphics Dojo 1.0')
TypeError: QNetworkRequest.setRawHeader(QByteArray, QByteArray):
argument 1 has unexpected type 'str'
Aborted
======================================================

Please update the example file for the next PyQt release.  Many thanks!

Cheers,

Anthony