fix: cla-backend/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-5805047
- https://snyk.io/vuln/SNYK-PYTHON-IDNA-6597975
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-1533435
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-5926907
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-6002459

cla-backend/requirements.txt

@@ -6,7 +6,7 @@ attrs==19.3.0
 beautifulsoup4==4.8.1
 boto3==1.9.236
 botocore==1.12.253
-certifi==2022.12.7
+certifi==2023.7.22
 chardet==3.0.4
 colorama==0.4.3
 coverage==4.5.4
@@ -19,7 +19,7 @@ future==0.18.3
 gossip==2.3.1
 gunicorn==19.9.0
 hug==2.6.0
-idna==2.8
+idna==3.7
 importlib-metadata==1.6.1
 Jinja2==2.11.3
 jmespath==0.9.4
@@ -53,7 +53,7 @@ six==1.13.0
 soupsieve==1.9.5
 termcolor==1.1.0
 typed-ast==1.4.1
-urllib3==1.25.11
+urllib3==1.26.18
 vintage==0.4.1
 wcwidth==0.1.7
 Werkzeug==0.15.5