Add astroid.objects.ExceptionInstance import

With typing added to parts of the code in ``pylint`` it has become useful 
to import this class so ``pylint`` can access it. See discussion in:
pylint-dev/pylint#4940

astroid/__init__.py

@@ -60,6 +60,7 @@
 from astroid.const import Context, Del, Load, Store
 from astroid.exceptions import *
 from astroid.inference_tip import _inference_tip_cached, inference_tip
+from astroid.objects import ExceptionInstance
 
 # isort: off
 # It's impossible to import from astroid.nodes with a wildcard, because