Fix daily stubtest error: Add find_distributions() to stubtest allowlist (#14879)

Author: srittau

stdlib/@tests/stubtest_allowlists/linux-py39.txt

@@ -6,6 +6,12 @@
 # while being inspected by stubtest. Fixed in Python 3.10.
 select.epoll.register
 
+# According to stubtest, these are not present at runtime, starting with
+# Python 3.9.24. Which is not true.
+_frozen_importlib_external.PathFinder.find_distributions
+importlib._bootstrap_external.PathFinder.find_distributions
+importlib.machinery.PathFinder.find_distributions
+
 
 # =============================================================
 # Allowlist entries that cannot or should not be fixed; <= 3.12