bake: Fix file write flushing problem with Python 3.8+ Writing the cache files fails in Python 3.8 because it looks like flushing behaviour has changed. We need to explicitly flush. And even then, in very rare occurrences, it looks like it can still run into racing conditions, so we do a very hacky and ugly "retry" loop when fetching cached data :(

---
url: /foo.html
theme_config:
    site:
        title: "Some Test Theme"
in:
    pages/foo.html: "This is: {{site.title}}"
out: "This is: Some Test Theme"
---
url: /foo.html
theme_config:
    site:
        title: "Some Test Theme"
in:
    pages/foo.html: "This is: {{foo}}"
    configs/theme_preview.yml: "foo: bar"
out: "This is: bar"