src_ds = gdal.Open('input.tif')
if src_ds is None:
    print ('Unable to open %s' % src_filename)
    sys.exit(1)
try:
    srcband = src_ds.GetRasterBand(1)
except RuntimeError:
    print ('Band 1 not found')
    sys.exit(1)

dst_layername = "output/path/filename"
drv = ogr.GetDriverByName("ESRI Shapefile")
if os.path.exists(dst_layername + ".shp"):
    drv.DeleteDataSource(dst_layername + ".shp")

# create spatial reference for the new vector based on the raster
srs = osr.SpatialReference()
srs.ImportFromWkt(src_ds.GetProjection())
dst_layer = dst_ds.CreateLayer(dst_layername, srs = srs)

# create polygon layer
dst_ds = drv.CreateDataSource(dst_layername + ".shp")
gdal.Polygonize(srcband, srcband, dst_layer, -1, [], callback=None)

### fix geometries
processing.run("native:fixgeometries",
        {'INPUT':"output/path/filename.shp",
        'OUTPUT':"output_fixed.shp"})
# > produces a file, but shows nothing