Add a NEWS entry.

Author: ericsnowcurrently

Misc/NEWS.d/next/Core and Builtins/2021-08-31-17-44-51.bpo-45020.ZPI_3L.rst

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+Freeze all the modules that are imported during Python startup.  This gives
+us meaningful performance improvements to startup.  Also add a new command
+line option, "-X frozen_modules=[on|off]" to opt out of (or into) using
+frozen modules.  This defaults to "on" (or "off" if running in a local
+development environment, saving contributors some frustration).