[3.8] bpo-37521: No longer treat insertion into sys.modules as optional in importlib...

[3.8] bpo-37521: No longer treat insertion into sys.modules as optional in importlib examples (GH-14723) (GH-14724)

Fix importlib examples to insert any newly created modules via importlib.util.module_from_spec() immediately into sys.modules instead of after calling loader.exec_module().

Thanks to Benjamin Mintz for finding the bug.

https://bugs.python.org/issue37521
(cherry picked from commit 0827064c955f88df8ba5621d8e3d81be3cfc26a9)

Co-authored-by: Brett Cannon <54418+brettcannon@users.noreply.github.com>
https://bugs.python.org/issue37521