Fix Travis GCC 7 Python 3.6.6 (pybind#1436)

Add missing python3 distutils on Debian "buster".

Author: ax3l

.travis.yml

@@ -108,7 +108,7 @@ before_install:
       export CXX=g++-$GCC CC=gcc-$GCC
     fi
     if [ "$GCC" = "6" ]; then DOCKER=${ARCH:+$ARCH/}debian:stretch
-    elif [ "$GCC" = "7" ]; then DOCKER=debian:buster EXTRA_PACKAGES+=" catch" DOWNLOAD_CATCH=OFF
+    elif [ "$GCC" = "7" ]; then DOCKER=debian:buster EXTRA_PACKAGES+=" catch python3-distutils" DOWNLOAD_CATCH=OFF
     fi
   elif [ "$TRAVIS_OS_NAME" = "osx" ]; then
     export CXX=clang++ CC=clang;