Add gunicorn as a dependency

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (e28af4f) 69.71% compared to head (3ebf88a) 70.75%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #63 +/- ## ========================================== + Coverage 69.71% 70.75% +1.04% ========================================== Files 7 7 Lines 383 383 ========================================== + Hits 267 271 +4 + Misses 116 112 -4 ``` | [Files](https://app.codecov.io/gh/NCATSTranslator/workflow-runner/pull/63?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [app/server.py](https://app.codecov.io/gh/NCATSTranslator/workflow-runner/pull/63?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-YXBwL3NlcnZlci5weQ==) | `80.62% <ø> (+2.32%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/NCATSTranslator/workflow-runner/pull/63/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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