base: Fix broken pygobject package

Ubuntu 20.04 ships with the `pygobject` Python package pre-installed
but does not include one of its dependencies: `pycairo` package.

This commit updates the base image Dockerfile to install the
`libcairo2-dev` system package, which is required by the `pycairo`
Python package, and re-install the `pygobject` Python package.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Dockerfile.base

@@ -45,6 +45,7 @@ RUN apt-get -y update && \
  help2man \
  iproute2 \
  lcov \
+ libcairo2-dev \
  libglib2.0-dev \
  libgtk2.0-0 \
  liblocale-gettext-perl \
@@ -110,6 +111,7 @@ RUN wget ${WGET_ARGS} https://github.com/Kitware/CMake/releases/download/v${CMAK
 # Install Python dependencies
 RUN python3 -m pip install -U pip && \
  pip3 install -U wheel setuptools && \
+ pip3 install pygobject && \
  pip3 install \
  -r https://raw.githubusercontent.com/zephyrproject-rtos/zephyr/master/scripts/requirements.txt \
  -r https://raw.githubusercontent.com/zephyrproject-rtos/mcuboot/master/scripts/requirements.txt \