astrodatagui: use relative imports for .ui file

import of custom widget only supported if run by main.py

main.py

@@ -0,0 +1,9 @@
+from PyQt5 import QtWidgets
+import sys
+
+from astrodatagui.AstrodataGUI import AstrodataGUI
+
+app = QtWidgets.QApplication(sys.argv)
+window = AstrodataGUI()
+
+app.exec_()