Linkcode bugfix (#6)

## Summary

Fix blob in github link URL generated by `linkcode.py` in documentation

### Why?

Links are currently broken due to a change in tag names.

### How?

Add a `v` in front of the tag.

## Checklist

Most checks are automated, but a few aren't, so make sure to go through
and tick them off, even if they don't apply. This checklist is here to
help, not deter you. Remember, "Slow is smooth, and smooth is fast".

- [x] **Unit tests**
  - Every input should have a test for it.
- Every potential raised exception should have a test ensuring it is
raised.
- [x] **Documentation**
  - New functions/classes/etc. must be added to `docs/api.rst`.
- Changed/added classes/methods/functions have appropriate
`versionadded`, `versionchanged`, or `deprecated`
[directives](http://www.sphinx-doc.org/en/stable/markup/para.html#directive-versionadded).
- The appropriate entry in `CHANGELOG.md` has been included in the
"Unreleased" section, i.e. "Added", "Changed", "Deprecated", "Removed",
"Fixed", or "Security".
- [x] **Future work**
- Future work should be documented in the contributor guide, i.e.
`.github/CONTRIBUTING.md`.

If you have any questions not answered by a quick readthrough of the
[contributor
guide](https://pysparkplug.mattefay.com/en/latest/contributor_guide.html),
add them to this PR and submit it.

docs/linkcode.py

@@ -35,7 +35,7 @@ def _get_blob() -> str:
     version = Version(version_str)
     if version.is_devrelease or version.is_postrelease:
         return _get_git_sha()
-    return version_str
+    return f"v{version_str}"
 
 
 def _get_git_sha() -> str: