Why so coarse?

Getting a notification for every change of a global in module, is likely to make use the use of global variables extremely expensive.

```
var = 0

CONST = 1
def foo(...):
    ...
```

I may well want to be notified if `foo` or `CONST` gets modified, but performance could suffer badly if we make a callback every time `var` is changed.

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What happens if a watched dictionary is modified in a callback?

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How do you plan to implement this? Steal a bit from `ma_version_tag` or replace `ma_version_tag`?
If you replace `ma_version_tag` this could actually speed things up a tad.

You'd probably need a PEP to replace PEP 509, but I think this may need a PEP anyway.