audit_events Sphinx extension appends to 'sources' list infinitely

[encukou]

Bug report

My Sphinx builds are taking a long time after a bunch of runs.
With @AA-Turner we tracked it down to the AuditEvents extension appending new values in each run to a list that's persisted in the environment pickle.

Adam, over to you :)

Linked PRs

• GH-134848: Use a set to store AuditEvents.sources #134849
• [3.14] GH-134848: Use a set to store AuditEvents.sources (GH-134849) #134853
• [3.13] GH-134848: Use a set to store AuditEvents.sources (GH-134849) #134854