types(promotion): add typing to convert

[nstarman]

@wesselb, this PR adds some type-check only annotations to convert. We can't do more runtime-accessible annotations without running into infinite recursion since convert is called to convert to the return type, unless it isn't annotated / annotated with Any.

[coveralls]

Pull Request Test Coverage Report for Build 12640304563

Details

• 6 of 8 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 99.609%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
plum/promotion.py	6	8	75.0%

Totals
Change from base Build 12033358902:	-0.2%
Covered Lines:	1275
Relevant Lines:	1280

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💛 - Coveralls

[nstarman]

I'm not sure why coveralls is reporting the TYPE_CHECKING block as unchecked.
In my projects this config woks great.
See https://coverage.readthedocs.io/en/latest/excluding.html#advanced-exclusion
NOTE: When you click on the coveralls, it shows the exclusion is working as expected.

[wesselb]

@nstarman, looks fantastic, as always :)! Merging right away. Thank you!!