ci: use buster image for py27 tests

3 jobs for topic/default/py27-buster in 47 seconds (queued for 3 seconds)
Name Stage Failure
failed
py27 Test
Requirement already satisfied: tox in /usr/lib/python3/dist-packages (3.7.0)
$ PATH=/usr/lib/postgresql/11/bin:${PATH} tox -e py27
GLOB sdist-make: /builds/cubicweb/cubes/datacat/setup.py
py27 create: /builds/cubicweb/cubes/datacat/.tox/py27
ERROR: Error creating virtualenv. Note that some special characters (e.g. ':' and unicode symbols) in paths are not supported by virtualenv. Error details: InvocationError('Failed to get version_info for python2.7: Traceback (most recent call last):\n File "<string>", line 1, in <module>\nImportError: No module named json\n', None)
___________________________________ summary ____________________________________
ERROR: py27: Error creating virtualenv. Note that some special characters (e.g. ':' and unicode symbols) in paths are not supported by virtualenv. Error details: InvocationError('Failed to get version_info for python2.7: Traceback (most recent call last):\n File "<string>", line 1, in <module>\nImportError: No module named json\n', None)
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1