Unpin Python setuptools dependency

Fixes #8285, allows us to work on later versions of virtualenv without
getting problems with too-recent python packaging versions.

tools/run_tests/build_python.sh

@@ -171,8 +171,7 @@ pip_install_dir() {
 }
 
 $VENV_PYTHON -m pip install --upgrade pip
-# TODO(https://github.com/pypa/setuptools/issues/709) get the latest setuptools
-$VENV_PYTHON -m pip install setuptools==25.1.1
+$VENV_PYTHON -m pip install setuptools
 $VENV_PYTHON -m pip install cython
 pip_install_dir $ROOT
 $VENV_PYTHON $ROOT/tools/distrib/python/make_grpcio_tools.py