fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-NUMPY-2321964
- https://snyk.io/vuln/SNYK-PYTHON-NUMPY-2321966
- https://snyk.io/vuln/SNYK-PYTHON-NUMPY-2321970
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-5537286

requirements.txt

@@ -16,4 +16,6 @@ Levenshtein
 retrying
 mailgw_temporary_email
 pycryptodome
-random-password-generator
+random-password-generator
+numpy>=1.22.2 # not directly required, pinned by Snyk to avoid a vulnerability
+tornado>=6.3.2 # not directly required, pinned by Snyk to avoid a vulnerability