[pygobject] CI: update pip in the xenial job

[Christoph Reiter <creiter src gnome org>]

commit ac3a69cb03bd526bd8f059681302f8802e9511a4
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Sat Feb 15 07:26:21 2020 +0100

    CI: update pip in the xenial job
    
    Some packages have started failing to install with the pip shipped
    with xenial.

 .gitlab-ci/test-docker-old.sh | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci/test-docker-old.sh b/.gitlab-ci/test-docker-old.sh
index d03d983d..c06f7ee2 100755
--- a/.gitlab-ci/test-docker-old.sh
+++ b/.gitlab-ci/test-docker-old.sh
@@ -16,6 +16,7 @@ mkdir -p "${COV_DIR}"
 mkdir -p "${CCACHE_DIR}"
 
 # test
+python -m pip install --upgrade pip
 python -m pip install pycairo pytest pytest-faulthandler coverage
 python setup.py build_tests
 xvfb-run -a python -m coverage run --context "${COV_KEY}" tests/runtests.py