add changelog entry

changelog/3016.bugfix

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+internal pytest test suite improvements:
+
+- cleaned up Testdir taking snapshots & restoring global Python state - extracted to new CwdSnapshot, SysModulesSnapshot & SysPathsSnapshot classes, each saving the state they are interested in on instantiation and restoring it in its restore() method
+- fixed restoring Python state after in-process pytest runs
+    - now each in-process pytest run saves a snapshot of important global Python state and restores it after the test completes, including the list of loaded modules & the Python path settings
+    - previously only the loaded package data was getting restored, but that was also reverting any loaded package changes done in the test triggering the pytest runs, and not only those done by the pytest runs themselves
+    - updated acceptance tests broken by this change, which were only passing before by accident as they were making multiple pytest runs with later ones depending on sys.path changes left behind by the initial one