Workaround to pip9 TLS1 certificate: manually bootstrap pip in macos

build

make/osx/build_wheels.sh

@@ -1,10 +1,10 @@
 set -e
 set +x
 
-for PYVERSION in 2.7 3.5 3.6 3.7; do
+for PYVERSION in 2.7 3.6 3.7; do
     virtualenv -p /Library/Frameworks/Python.framework/Versions/${PYVERSION}/bin/python${PYVERSION} venv_${PYVERSION}
     . ./venv_${PYVERSION}/bin/activate
-    pip install pip --upgrade
+    curl https://bootstrap.pypa.io/get-pip.py | python
     pip install -r requirements/setup.txt
     python setup.py bdist_wheel
     pip install -r requirements/test.txt