Sphinx docs build in CI produces warning due to import error when importing httomo.cli module

[yousefmoazzam]

See this CI job (in the `Build docs" section) for the warning, the main point of interest is the following:

/home/runner/work/httomo/httomo/docs/source/developers/api.rst:4: WARNING: autosummary: failed to import httomo.cli.
Possible hints:
* AttributeError: module 'httomo' has no attribute 'cli'
* ImportError: cannot import name '__version__' from 'httomo' (/home/runner/work/httomo/httomo/httomo/__init__.py)
* ImportError: 

As mentioned in #494, this isn't seen when building the docs locally, only when building the docs in CI, so this may be trickier to understand the cause...