fix: tests/milvus_python_test/requirements_cluster.txt to reduce vulnerabilities (#2221)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYLINT-568073

tests/milvus_python_test/requirements_cluster.txt

@@ -9,7 +9,7 @@ more-itertools==7.0.0
 numpy==1.16.3
 pluggy==0.12.0
 py==1.8.0
-pylint==2.3.1
+pylint==2.5.0
 pytest==4.5.0
 pytest-timeout==1.3.3
 pytest-repeat==0.8.0