[IMP] mail_tracking: email score performance

[chienandalu]

In instances with tons of trackings this method can resent performance. Hopefully this improves it.

cc @Tecnativa

[yajo]

Remove the surrounding [...] brackets.

With brackets:

1. Produce a lazy generator.
2. Exhaust the generator converting it into a list.
3. Convert the list in a dict.

Without brackets:

1. Produce a lazy generator.
2. Exhaust the generator converting it into a dict.

You can save tons of loops! 😊

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Note that another idiomatic way (equivalent to doing it without brackets) to do so is:

mapped_data = {state['state']: state['state_count'] for state in data}

[chienandalu]

Noted and fixed 😉

Full coverage as well.