bpo-45020: Default to using frozen modules unless running from source tree. (gh-28940)

bpo-45020: Default to using frozen modules unless running from source tree. (gh-28940)

The default was "off".  Switching it to "on" means users get the benefit of frozen stdlib modules without having to do anything.  There's a special-case for running-in-source-tree, so contributors don't get surprised when their stdlib changes don't get used.

https://bugs.python.org/issue45020