Allow parallel execution

[giampaolo]

PR Description

Properly fix #297.
For the time being, I'm using this monkey patch in my own conf.py:

def setup(app):
    # ablog doesn't implement env-merge-info so it's not parallel-read safe.
    # Patch it here: merge worker envs into the main one, then declare it safe.
    def _merge_ablog_posts(app, env, docnames, other):
        if hasattr(other, "ablog_posts"):
            if not hasattr(env, "ablog_posts"):
                env.ablog_posts = {}
            env.ablog_posts.update(other.ablog_posts)

    app.connect("env-merge-info", _merge_ablog_posts)
    app.extensions["ablog"].parallel_read_safe = True

This PR would avoid that, and allow parallel execution via python3 -m sphinx -b html --jobs=auto . _build/html

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[SilverRainZ]

I am not ABlog developer, just a user like you. I think you can update src/ablog/tests/test_parallel.py?