fix: tests/milvus_python_test/requirements.txt to reduce vulnerabilities (#2224)

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

tests/milvus_python_test/requirements.txt

 numpy>=1.18.0
-pylint==2.3.1
+pylint==2.5.0
 pytest==4.5.0
 pytest-timeout==1.3.3
 pytest-repeat==0.8.0