Add QApplication check to init_venv.py.

If no XServer is available, importing QtWidgets will work, but Qt will abort
when trying to construct a QApplication.

scripts/init_venv.py

@@ -119,6 +119,9 @@ def test_toolchain():
             pkg = 'bs4'
         print("Importing {}".format(pkg))
         venv_python('-c', 'import {}'.format(pkg))
+    print("Checking if we can use QApplication")
+    venv_python('-c',
+                'from PyQt5.QtWidgets import QApplication; QApplication([])')
 
 
 def verbose_copy(src, dst, *, follow_symlinks=True):