add license identifiers to files missing them

src/main/python/constants.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 KEY_WIDTH = 40
 KEY_HEIGHT = KEY_WIDTH
 KEY_SPACING = 4

src/main/python/keyboard_container.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 from collections import defaultdict
 
 from PyQt5.QtCore import pyqtSignal, Qt

src/main/python/main_window.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 from PyQt5.QtCore import Qt
 from PyQt5.QtWidgets import QWidget, QComboBox, QToolButton, QHBoxLayout, QVBoxLayout
 

src/main/python/tabbed_keycodes.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 from PyQt5.QtCore import Qt, pyqtSignal
 from PyQt5.QtWidgets import QTabWidget, QWidget, QPushButton, QScrollArea