Fix version extraction in Sphinx config

[vloncar]

Description

With the merging of #479 the location of __version__ changed and Sphinx config still tries to extract it from hls4ml/__init__.py. This changes the config of Sphinx to extract the version information from the new _version.py.

Type of change

• Bug fix (non-breaking change that fixes an issue)

[jmduarte]

@vloncar what a nightmare trying to get the sphinx-action GitHub action to work. Let me summarize:

• We need git available in the image because that's how setuptools_scm gets the version (through git tag etc.)
• It turns out that because https://github.com/ammaraskar/sphinx-action is not really being maintained right now, you can't install git via apt install git as part of the pre-build-command step due to a variety of other issues (outdated image; trying to install using os.system() from python, etc. see e.g. Cannot run apt update  ammaraskar/sphinx-action#33). There are proposed solutions, but because it's not being maintained, nobody is accepting the PRs, etc.
• I forked it and just installed git in the Dockerfile

Now it works.

[vloncar]

@jmduarte I fixed the failing test of reading h5 file. It failed in quartus for unrelated reason, due to the name of the project not being propagated. Ready to (squash and) merge now?