fix-RTD-build-error

[juhoinkinen]

Although the RTD build errors seemed already to be fixed by adding .readthedocs.yml (in #285), the errors still appeared (first in the stable version build, then also in latest.

The errors seem to be related to capitalization of package names, the actual error is:

error: The 'click' distribution was not found and is required by click-log, annif

The same error raised also for flask (after trying to get pass the click error).

See also the discussion in readthedocs/readthedocs.org#4051

On my local machine a workaround for these is to add flask and PyYAML packages to the docs/requirements.txt

[juhoinkinen]

Build for stable version still fails due to the same error, but this is expected as that build is based on the current release (Annif 0.41) and will be fixed by the next release.