Merge pull request #25260 from donnadionne/build_python

Modify build_python.sh because get-pip.py has moved.

tools/run_tests/helper_scripts/build_python.sh

@@ -153,7 +153,7 @@ fi
 # 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.