(ci) Use python3 instead of python2

Author: frazar

.travis.yml

@@ -35,11 +35,12 @@ before_install:
   #
   # PlatformIO setup
   - uname --all
-  - sudo apt-get update -qq && sudo apt-get -y install python-pip
-  - python2 --version
-  - pip2 install --user --upgrade pip
-  - pip2 --version
-  - pip2 install --user --upgrade platformio
+  - sudo apt-get update -qq && sudo apt-get -y install python3-pip
+  - python3 --version
+  - pip3 install --user --upgrade pip
+  - pip3 --version
+  - pip3 install --user --upgrade setuptools
+  - pip3 install --user --upgrade platformio
   - platformio update
   - platformio --version
   #