fix: server/predictors/chemprop/requirements.txt to reduce vulnerabil…

…ities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-REQUESTS-6928867

server/predictors/chemprop/requirements.txt

@@ -13,3 +13,4 @@ tensorflow>=1.13.1
 torch>=1.5.1
 tqdm>=4.45.0
 typed-argument-parser>=1.5.2
+requests>=2.32.0 # not directly required, pinned by Snyk to avoid a vulnerability