travis: install flake8 using pip3

this ensure no python2 flake8 will be installed and prevent confusion with required python3 flake8

.travis.yml

 dist: trusty
 
 before_install:
-  - sudo apt-get install coreutils realpath doxygen graphviz python-lesscpy cppcheck coccinelle pcregrep python3-flake8
+  - sudo apt-get install coreutils realpath doxygen graphviz python-lesscpy cppcheck coccinelle pcregrep python3-pip
+  - sudo pip3 install flake8
 
 script:
   - make static-test