fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-7267250

requirements.txt

@@ -4,3 +4,4 @@ img2pdf~=0.5.1
 requests~=2.31.0
 ConfigArgParse~=1.7
 pillow>=10.3.0
+urllib3>=2.2.2 # not directly required, pinned by Snyk to avoid a vulnerability