Derement an item

[nyu-kaitlyn]

This will decrease the quantity of items by one or delete the item if the quantity is 1 already

[codecov[bot]]

Codecov Report

Patch coverage: 92.59% and project coverage change: -0.35 :warning:

Comparison is base (bf4218d) 95.19% compared to head (3d49955) 94.85%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #74 +/- ## ========================================== - Coverage 95.19% 94.85% -0.35% ========================================== Files 8 8 Lines 333 369 +36 ========================================== + Hits 317 350 +33 - Misses 16 19 +3 ``` | [Impacted Files](https://codecov.io/gh/CSCI-GA-2820-SP23-001/shopcarts/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SP23-001) | Coverage Δ | | |---|---|---| | [service/routes.py](https://codecov.io/gh/CSCI-GA-2820-SP23-001/shopcarts/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SP23-001#diff-c2VydmljZS9yb3V0ZXMucHk=) | `94.85% <92.59%> (-1.31%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://codecov.io/gh/CSCI-GA-2820-SP23-001/shopcarts/pull/74/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SP23-001) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SP23-001). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SP23-001)

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[ctn8176]

woohoo solid code!