Pin to 4.3 instead of 4.0

requirements.bazel.txt

@@ -14,5 +14,5 @@ chardet==3.0.4
 certifi==2017.4.17
 idna==2.7
 googleapis-common-protos==1.5.5
-# rsa 4.0 is the last version support Python 2
+# rsa 4.3 is the last version support Python 2
-rsa==4.0
+rsa==4.3

requirements.txt

@@ -5,5 +5,5 @@ enum34>=1.0.4
 protobuf>=3.5.0.post1
 six>=1.10
 wheel>=0.29
-# rsa 4.0 is the last version support Python 2
+# rsa 4.3 is the last version support Python 2
-rsa==4.0
+rsa==4.3

tools/run_tests/helper_scripts/build_python.sh

@@ -222,6 +222,9 @@ pip_install_dir "$ROOT/src/python/grpcio_status"
 # Install testing
 pip_install_dir "$ROOT/src/python/grpcio_testing"
 
+# rsa 4.3 is the latest version support Python 2
+pip_install ras==4.3
+
 # Build/install tests
 pip_install coverage==4.4 oauth2client==4.1.0 \
             google-auth==1.17.1 requests==2.14.2 \