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This replaces the previous QSTR_null entry in the globals dict which could
leak out to Python (e.g. via iteration of mod.__dict__) and could lead to
crashes.

It results in smaller code size at the expense of turning a lookup into a
loop, but the list it is looping over likely only contains one or two
elements.

To allow a module to register its custom attr function it can use the new
`MP_REGISTER_MODULE_DELEGATION` macro.

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>