PyUtils: Update sequence check in PyROOTInspector

This is discussed a bit in the context of ATEAM-974 and root/issues/15161. It seems like the most recent cppyy versions unconditionally define __getitem__ for all classes, which causes PySequence_Check to return true(docs). This MR introduces a custom method that checks if an object is a sequence by attempting to get the first item. This is not a 1-to-1 replacement but should work for all practical purposes here. Let me tag @ssnyder in case he wants to comment.

Related to ATEAM-974