update ubuntu bionic to use python 3.7

The pip package is for python 3.6, but installs pip for all versions of
python. Apparently. Including python 3.7.

So do all other packages, especially the ones where it doesn't work but
pip thinks it is installed anyway. Force a reinstall.

ci/ciimage/bionic/install.sh

@@ -10,6 +10,7 @@ export DC=gdc
 
 pkgs=(
   python3-pip libxml2-dev libxslt1-dev libyaml-dev libjson-glib-dev
+  python3.7 python3.7-dev
   wget unzip cmake doxygen
   clang
   pkg-config-arm-linux-gnueabihf
@@ -47,7 +48,13 @@ done
 # packages
 eatmydata apt-get -y install "${pkgs[@]}"
 
-install_python_packages
+# Actually select the right python version
+update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1 \
+    --slave /usr/lib/x86_64-linux-gnu/pkgconfig/python3.pc python3.pc /usr/lib/x86_64-linux-gnu/pkgconfig/python-3.6.pc
+update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 2 \
+    --slave /usr/lib/x86_64-linux-gnu/pkgconfig/python3.pc python3.pc /usr/lib/x86_64-linux-gnu/pkgconfig/python-3.7.pc
+
+python3 -m pip install -U "${base_python_pkgs[@]}" "${python_pkgs[@]}"
 
 # Install the ninja 0.10
 wget https://github.com/ninja-build/ninja/releases/download/v1.10.0/ninja-linux.zip