Force version of python's module on xenial.

Xenial's python is 3.5 and recent versions of used python modules are
not supporting python 3.5 anymore.
We have to force specific versions for gcovr and its dependencies.

.github/ci_images/xenial_builder.dockerfile

@@ -21,7 +21,7 @@ RUN apt update -q \
 #    vim less grep \
   && apt-get clean -y \
   && rm -rf /var/lib/apt/lists/* /usr/share/doc/* /var/cache/debconf/* \
-  && pip3 install meson==0.52.1 pytest gcovr requests distro
+  && pip3 install meson==0.52.1 pytest 'markupsafe<2.0.0' 'jinja2<3.0.0' 'gcovr<5.0' requests distro
 
 # Create user
 RUN useradd --create-home runner