feat(python): read version-file from manifest

[fredrikaverpil]

Why this change?

I wanted to write the version string to __about__.py instead of the default __init__.py file for release-type python. Writing the version string to __about__.py is quite common in the python community.

After fiddling around, I noticed that the version-file value from the manifest JSON is not read and could've been used so to specify the path to __about__.py.

What was changed?

• Read the manifest's version-file value and push the version string to it.
• If no version-file was provided, fall back to previous behavior.

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Fixes #2433

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