fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-3164749
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-5805047
- https://snyk.io/vuln/SNYK-PYTHON-IDNA-6597975
- https://snyk.io/vuln/SNYK-PYTHON-PYJWT-2840625
- https://snyk.io/vuln/SNYK-PYTHON-REQUESTS-5595532

requirements.txt

@@ -1,6 +1,9 @@
 mock
 pytest==5.4.3
 six
-requests>=2.24.0
+requests>=2.31.0
 unicodecsv>=0.14.1
 simple-salesforce>=1.0.0
+certifi>=2023.7.22 # not directly required, pinned by Snyk to avoid a vulnerability
+idna>=3.7 # not directly required, pinned by Snyk to avoid a vulnerability
+pyjwt>=2.4.0 # not directly required, pinned by Snyk to avoid a vulnerability