[travis] add python3 to continuous integration (#3146)

.travis.yml

@@ -26,7 +26,8 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 
-language: generic
+language: python
+python: "3.6"
 
 sudo: required
 dist: trusty
@@ -58,6 +59,7 @@ matrix:
       compiler: gcc
     - env: BUILD_TARGET="android-build" VERBOSE=1
       os: linux
+      python: "2.7"
     - env: BUILD_TARGET="posix-distcheck" CC=clang-6.0 CXX=clang++-6.0 VERBOSE=1 VIRTUAL_TIME=1
       os: linux
       addons:
@@ -70,6 +72,7 @@ matrix:
     - env: BUILD_TARGET="posix-32-bit" VERBOSE=1 VIRTUAL_TIME=1
       os: linux
       compiler: gcc
+      python: "2.7"
     - env: BUILD_TARGET="posix-ncp" VERBOSE=1 VIRTUAL_TIME=1
       os: linux
       compiler: gcc
@@ -162,4 +165,5 @@ matrix:
       compiler: gcc
     - env: BUILD_TARGET="toranj-test-framework" VERBOSE=1 TORANJ_POSIX_APP_RCP_MODEL=1
       os: linux
-      compiler: gcc
+      compiler: gcc
+      python: "2.7"

.travis/before_install.sh

@@ -40,11 +40,11 @@ cd /tmp || die
     sudo apt-get update || die
 
     [ $BUILD_TARGET != posix-distcheck -a $BUILD_TARGET != posix-32-bit -a $BUILD_TARGET != posix-app-cli -a $BUILD_TARGET != posix-mtd -a $BUILD_TARGET != posix-ncp -a $BUILD_TARGET != posix-app-ncp ] || {
-        pip install --user --upgrade pip || die
-        pip install --user -r $TRAVIS_BUILD_DIR/tests/scripts/thread-cert/requirements.txt || die
+        pip install --upgrade pip || die
+        pip install -r $TRAVIS_BUILD_DIR/tests/scripts/thread-cert/requirements.txt || die
         [ $BUILD_TARGET != posix-ncp -a $BUILD_TARGET != posix-app-ncp ] || {
             # Packages used by ncp tools.
-            pip install --user git+https://github.com/openthread/pyspinel || die
+            pip install git+https://github.com/openthread/pyspinel || die
         }
     }
 

.travis/script.sh

@@ -34,6 +34,8 @@ die() {
 
 set -x
 
+python --version || die
+
 [ $BUILD_TARGET != pretty-check ] || {
     ./bootstrap || die
     ./configure || die