Fix iOS binary size build

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -67,7 +67,7 @@ then
   time git clone --depth 1 https://github.com/CocoaPods/Specs.git ~/.cocoapods/repos/master
 
   # Needed for ios-binary-size
-  time pip install --user pyyaml pyjwt pyOpenSSL cryptography requests
+  time pip install --user pyyaml pyjwt==1.7.1 pyOpenSSL cryptography requests
 
   # Store intermediate build files of ObjC tests into /tmpfs
   # TODO(jtattermusch): this has likely been done to avoid running
@@ -85,7 +85,7 @@ if [ "${PREPARE_BUILD_INSTALL_DEPS_PYTHON}" == "true" ]
 then
   # python
   time pip install --user virtualenv
-  time pip install --user --upgrade Mako tox setuptools==44.1.1 twisted pyyaml pyjwt pyOpenSSL cryptography requests
+  time pip install --user --upgrade Mako tox setuptools==44.1.1 twisted pyyaml pyjwt==1.7.1 pyOpenSSL cryptography requests
 
   # Install Python 3.7 if it doesn't exist
   if [ ! -f "/usr/local/bin/python3.7" ]; then