Modify build_python.sh because get-pip.py has moved.
diff --git a/tools/run_tests/helper_scripts/build_python.sh b/tools/run_tests/helper_scripts/build_python.sh
index 331f10e..b437eb3 100755
--- a/tools/run_tests/helper_scripts/build_python.sh
+++ b/tools/run_tests/helper_scripts/build_python.sh
@@ -153,7 +153,7 @@
# See https://github.com/grpc/grpc/issues/14815 for more context. We cannot rely
# on pip to upgrade itself because if pip is too old, it may not have the required
# TLS version to run `pip install`.
-curl https://bootstrap.pypa.io/get-pip.py | $VENV_PYTHON
+curl https://bootstrap.pypa.io/2.7/get-pip.py | $VENV_PYTHON
# pip-installs the directory specified. Used because on MSYS the vanilla Windows
# Python gets confused when parsing paths.