[pygobject/pygobject-3-36] CI: make sure we install a Python 3.5 compatible pip

[Christoph Reiter <creiter src gnome org>]

commit e77860549dc75dd2e228eb684ca6cd404896d50f
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Mon Feb 1 20:32:48 2021 +0100

    CI: make sure we install a Python 3.5 compatible pip
    
    pip dropped <=3.5 support in the latest release

 .gitlab-ci/test-docker-old.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci/test-docker-old.sh b/.gitlab-ci/test-docker-old.sh
index c06f7ee2..5cb4a907 100755
--- a/.gitlab-ci/test-docker-old.sh
+++ b/.gitlab-ci/test-docker-old.sh
@@ -16,7 +16,7 @@ mkdir -p "${COV_DIR}"
 mkdir -p "${CCACHE_DIR}"
 
 # test
-python -m pip install --upgrade pip
+python -m pip install --upgrade 'pip<21'
 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