fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-FONTTOOLS-6133203
- 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-PILLOW-5918878
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6043904
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6182918
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6219984
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6219986
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6514866

requirements.txt

@@ -2,3 +2,6 @@ sycret
 syft @ git+https://github.com/OpenMined/PySyft@sympc-dev#egg=syft&subdirectory=packages/syft
 torch>=1.6,<=1.8.1
 torchcsprng<=0.2.1
+fonttools>=4.43.0 # not directly required, pinned by Snyk to avoid a vulnerability
+numpy>=1.22.2 # not directly required, pinned by Snyk to avoid a vulnerability
+pillow>=10.3.0 # not directly required, pinned by Snyk to avoid a vulnerability