fix: requirements.txt to reduce vulnerabilities.

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-1727377

Author: snyk-bot

requirements.txt

@@ -9,6 +9,6 @@ Flask==1.1.2
 Werkzeug==2.0.3
 tqdm==4.54.1
 olefile==0.46
-Pillow==8.1.2
+Pillow==8.3.2
 py-bcrypt==0.4
 scikit_learn==0.22.2.post1