SPIRV-Tools requires python3 (#2510)

Updates:
- CMake configuration
- Kokoro build scripts on Linux and macos (Windows already uses Python 3.6)

CMakeLists.txt

@@ -178,7 +178,8 @@ if(NOT COMMAND find_host_program)
   endmacro()
 endif()
 
-find_host_package(PythonInterp)
+# Tests require Python3
+find_host_package(PythonInterp 3 REQUIRED)
 
 # Check for symbol exports on Linux.
 # At the moment, this check will fail on the OSX build machines for the Android NDK.

README.md

@@ -276,14 +276,8 @@ installed regardless of your OS:
 
 - [CMake](http://www.cmake.org/): for generating compilation targets.  Version
   2.8.12 or later.
-- [Python](http://www.python.org/): for utility scripts and running the test 
-suite. Version 2 or 3.
-
-We will be moving to Python3 only in the future.  If you are using Python2, you
-will need to install Python-future: 
-```
-pip install future
-```
+- [Python 3](http://www.python.org/): for utility scripts and running the test
+suite.
 
 SPIRV-Tools is regularly tested with the the following compilers:
 

kokoro/scripts/linux/build.sh

@@ -77,7 +77,7 @@ mkdir build && cd $SRC/build
 # Invoke the build.
 BUILD_SHA=${KOKORO_GITHUB_COMMIT:-$KOKORO_GITHUB_PULL_REQUEST_COMMIT}
 echo $(date): Starting build...
-cmake -DPYTHON_EXECUTABLE:FILEPATH=/usr/bin/python2.7 -GNinja -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_INSTALL_PREFIX=install -DRE2_BUILD_TESTING=OFF $ADDITIONAL_CMAKE_FLAGS $CMAKE_C_CXX_COMPILER ..
+cmake -DPYTHON_EXECUTABLE:FILEPATH=/usr/bin/python3 -GNinja -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_INSTALL_PREFIX=install -DRE2_BUILD_TESTING=OFF $ADDITIONAL_CMAKE_FLAGS $CMAKE_C_CXX_COMPILER ..
 
 echo $(date): Build everything...
 ninja

kokoro/scripts/macos/build.sh

@@ -41,7 +41,14 @@ mkdir build && cd $SRC/build
 # Invoke the build.
 BUILD_SHA=${KOKORO_GITHUB_COMMIT:-$KOKORO_GITHUB_PULL_REQUEST_COMMIT}
 echo $(date): Starting build...
-cmake -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=$BUILD_TYPE ..
+# We need Python 3.  At the moment python3.7 is the newest Python on Kokoro.
+cmake \
+  -GNinja \
+  -DPYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3.7 \
+  -DCMAKE_C_COMPILER=clang \
+  -DCMAKE_CXX_COMPILER=clang++ \
+  -DCMAKE_BUILD_TYPE=$BUILD_TYPE \
+  ..
 
 echo $(date): Build everything...
 ninja