Merge pull request #3022 from erwincoumans/master

trying to fix travis builds

.ci/docker/ubuntu-bionic

@@ -13,6 +13,7 @@ RUN apt-get install -y \
     lsb-release \
     pkg-config \
     python3 \
+    python3-dev \
     python3-distutils \
     software-properties-common \
     sudo

.ci/docker/ubuntu-xenial

@@ -13,5 +13,6 @@ RUN apt-get install -y \
     lsb-release \
     pkg-config \
     python3 \
+    python3-dev \
     software-properties-common \
     sudo

.ci/script.sh

@@ -7,6 +7,7 @@ if [[ "$TRAVIS_OS_NAME" == "linux" && "$CXX" = "g++" ]]; then
   $SUDO apt-get update
   $SUDO apt-get install -y python3
   $SUDO apt-get install -y python3-pip
+  $SUDO apt-get install python3-dev
   $SUDO pip3 install -U wheel
   $SUDO pip3 install -U setuptools
   python3 setup.py install --user