fix(examples) import 'u'-prefixed versions of modules (#2365)

import usys and utime instead of sys and time, as the latter are not available on all platforms

examples/widgets/chart/lv_example_chart_7.py

@@ -1,5 +1,5 @@
 #!/opt/bin/lv_micropython -i
-import time
+import utime as time
 import lvgl as lv
 import display_driver