fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-FUTURE-3180414

requirements.txt

@@ -28,7 +28,7 @@ fastavro==0.23.6          # via apache-beam
 fasteners==0.16           # via google-apitools
 git+git://github.com/oppia/firebase-admin-python@8d6deee889733d09ef5327d78c09e4d7b17785ac#egg=firebase-admin  # via -r requirements.in
 funcsigs==1.0.2           # via apache-beam, mock
-future==0.18.2            # via -r requirements.in, apache-beam
+future==0.18.3            # via -r requirements.in, apache-beam
 futures==3.3.0            # via apache-beam, google-api-core, grpcio, pyarrow
 google-api-core[grpc,grpcgcp]==1.22.4  # via firebase-admin, google-api-python-client, google-cloud-bigquery, google-cloud-bigtable, google-cloud-core, google-cloud-datastore, google-cloud-dlp, google-cloud-firestore, google-cloud-language, google-cloud-pubsub, google-cloud-spanner, google-cloud-tasks, google-cloud-videointelligence, google-cloud-vision
 google-api-python-client==1.12.8  # via firebase-admin