Pin pip to 19.3.1 to fix the build failure on master

tools/dockerfile/distribtest/python_dev_centos7_x64/Dockerfile

@@ -18,7 +18,7 @@ RUN yum install -y python
 RUN yum install -y python-devel
 RUN yum install -y epel-release
 RUN yum install -y python-pip
-RUN pip install --upgrade pip
+RUN pip install --upgrade pip==19.3.1
 RUN pip install -U virtualenv
 
 # The default gcc of CentOS 7 is gcc 4.8 which is older than gcc 4.9,

tools/internal_ci/linux/grpc_xds_bazel_python_test_in_docker.sh

@@ -25,7 +25,7 @@ cd /var/local/git/grpc
 VIRTUAL_ENV=$(mktemp -d)
 virtualenv "$VIRTUAL_ENV"
 PYTHON="$VIRTUAL_ENV"/bin/python
-"$PYTHON" -m pip install --upgrade pip
+"$PYTHON" -m pip install --upgrade pip==19.3.1
 "$PYTHON" -m pip install --upgrade grpcio-tools google-api-python-client google-auth-httplib2 oauth2client
 
 # Prepare generated Python code.

tools/internal_ci/linux/grpc_xds_bazel_test_in_docker.sh

@@ -25,7 +25,7 @@ cd /var/local/git/grpc
 VIRTUAL_ENV=$(mktemp -d)
 virtualenv "$VIRTUAL_ENV"
 PYTHON="$VIRTUAL_ENV"/bin/python
-"$PYTHON" -m pip install --upgrade pip
+"$PYTHON" -m pip install --upgrade pip==19.3.1
 "$PYTHON" -m pip install --upgrade grpcio grpcio-tools google-api-python-client google-auth-httplib2 oauth2client
 
 # Prepare generated Python code.

tools/internal_ci/linux/grpc_xds_csharp_test_in_docker.sh

@@ -25,7 +25,7 @@ cd /var/local/git/grpc
 VIRTUAL_ENV=$(mktemp -d)
 virtualenv "$VIRTUAL_ENV"
 PYTHON="$VIRTUAL_ENV"/bin/python
-"$PYTHON" -m pip install --upgrade pip
+"$PYTHON" -m pip install --upgrade pip==19.3.1
 "$PYTHON" -m pip install --upgrade grpcio grpcio-tools google-api-python-client google-auth-httplib2 oauth2client
 
 # Prepare generated Python code.

tools/internal_ci/linux/grpc_xds_php_test_in_docker.sh

@@ -25,7 +25,7 @@ cd /var/local/git/grpc
 VIRTUAL_ENV=$(mktemp -d)
 virtualenv "$VIRTUAL_ENV"
 PYTHON="$VIRTUAL_ENV"/bin/python
-"$PYTHON" -m pip install --upgrade pip
+"$PYTHON" -m pip install --upgrade pip==19.3.1
 "$PYTHON" -m pip install --upgrade grpcio-tools google-api-python-client google-auth-httplib2 oauth2client
 
 # Prepare generated Python code.

tools/internal_ci/linux/grpc_xds_ruby_test_in_docker.sh

@@ -25,7 +25,7 @@ cd /var/local/git/grpc
 VIRTUAL_ENV=$(mktemp -d)
 virtualenv "$VIRTUAL_ENV"
 PYTHON="$VIRTUAL_ENV"/bin/python
-"$PYTHON" -m pip install --upgrade pip
+"$PYTHON" -m pip install --upgrade pip==19.3.1
 "$PYTHON" -m pip install --upgrade grpcio-tools google-api-python-client google-auth-httplib2 oauth2client
 
 # Prepare generated Python code.

tools/run_tests/artifacts/build_package_python.sh

@@ -24,7 +24,7 @@ mkdir -p artifacts/
 cp -r "${EXTERNAL_GIT_ROOT}"/input_artifacts/python_*/* artifacts/ || true
 
 apt-get install -y python-pip
-python -m pip install -U pip
+python -m pip install -U pip==19.3.1
 python -m pip install -U wheel
 
 strip_binary_wheel() {