Fix in defaults.py preventing AttributeError

[ifrh]

Depending on python versions running "./src/manage-test.py" failed with

src/settings/defaults.py", line 23, in load_defaults
    if not isinstance(v, collections.abc.Callable) and not k.startswith('__'):
AttributeError: 'module' object has no attribute 'abc'

i.e. Travis run from 7th May 2022 failed : https://app.travis-ci.com/github/ifrh/Praktomat/jobs/569431166

This fix works: see https://app.travis-ci.com/github/ifrh/Praktomat/builds/250348409

[hannesbraun]

I wonder if this is really necessary as this should only occur with versions of Python prior to 3.3. The README states that Python 2 won't be supported anymore (makes sense). I don't know if it's worth to add specific code targeting problems on Python 3 versions that are older than 10 years as of now. I'd say no as newer versions of Python should be available on every recent system.