Override pip and virtualenv installation for macOS. (#7704)

This commit cleans up our environment setup for macOS builds.

-  Always use `python -m {pip | virtualenv}` form of commands.
-  Only upgrade in-place, and never use `--ignore-installed`.
-  Use latest tox on macos, and test 3.6 on macos presubmit.

These changes ensure that multibuilder won't break the system-installed Python on the Kokoro macOS builder, and thus won't resolve the ancient system version. For Tox, make sure we test with Python 3.6 (the version of Python in current macOS).

kokoro/macos/prepare_build_macos_rc

@@ -66,7 +66,7 @@ fi
 # Install Tox
 
 if [[ "${KOKORO_INSTALL_TOX:-}" == "yes" ]] ; then
-  sudo pip install tox==2.4.1
+  sudo python3 -m pip install --upgrade pip tox
 fi
 
 ##

kokoro/release/python/macos/config.sh

@@ -1,6 +1,25 @@
 # Define custom utilities
 # Test for OSX with [ -n "$IS_OSX" ]
 
+function remove_travis_ve_pip {
+    # Removing the system virtualenv or pip can be very problematic for
+    # macOS on Kokoro, so just leave them be.
+    :;
+}
+
+function install_pip {
+    check_python
+    PIP_CMD="sudo $PYTHON_EXE -m pip${pip_args:+ $pip_args}"
+    $PIP_CMD install --upgrade pip
+}
+
+function install_virtualenv {
+    check_python
+    check_pip
+    $PIP_CMD install --upgrade virtualenv
+    VIRTUALENV_CMD="$PYTHON_EXE -m virtualenv"
+}
+
 function pre_build {
     # Any stuff that you need to do before you start building the wheels
     # Runs in the root directory of this repository.

tests.sh

@@ -314,7 +314,7 @@ build_python() {
   if [ $(uname -s) == "Linux" ]; then
     envlist=py\{27,33,34,35,36\}-python
   else
-    envlist=py27-python
+    envlist=py\{27,36\}-python
   fi
   tox -e $envlist
   cd ..
@@ -364,7 +364,7 @@ build_python_cpp() {
   if [ $(uname -s) == "Linux" ]; then
     envlist=py\{27,33,34,35,36\}-cpp
   else
-    envlist=py27-cpp
+    envlist=py\{27,36\}-cpp
   fi
   tox -e $envlist
   cd ..