from PyQt5.QtWidgets import (
QApplication,
QWidget,
QLabel,
QMainWindow,
QHBoxLayout,
QSizePolicy
)
from PyQt5.QtCore import Qt
app = QApplication([])
window = QMainWindow()
container = QWidget()
container.setLayout(QHBoxLayout())
label_centerer = QMainWindow()
#Acá hago el cambio a widget
label_centerer.setWindowFlags(Qt.Widget)
label = QLabel("hola mundo")
label.setStyleSheet("background: gray")
label_centerer.setCentralWidget(label)
container.layout().addWidget(label_centerer)