python travis2bash updates

- ignores things like "_with_system_site_packages"
- allows to skip python versions by using TRAVIS_PYTHON environment
  variable (set it to the desired python version to skip others)

travis2bash.sh

@@ -158,7 +158,12 @@ if [[ "$(declare -p travis_python)" =~ "declare -a" ]]; then
         # strip quotes
         temp="${pv%\"}"
         pv="${temp#\"}"
+        # strip crap
+        pv=$(echo "$pv"|cut -d_ -f1)
         ppath=/usr/bin/python$pv
+        if [ -n "$TRAVIS_PYTHON" ] && [ "$TRAVIS_PYTHON" != "$pv" ]; then
+            echo "skipping python-$pv due to TRAVIS_PYTHON=$TRAVIS_PYTHON"
+        fi
         #if [ ! -e "$ppath" ]; then
         sudo apt -y install "python$pv" "python${pv}-dev"
         #fi