add_js_file(..., async="async") causes SyntaxError

[ftnext]

Is your feature request related to a problem? Please describe.
I want to put <script src="..." async="async"></script> tag in built HTML.
I found the example in https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_js_file and I tried it.

In conf.py,

def setup(app):
    app.add_js_file('example.js', async="async")

$ make html
Sphinx v4.3.1 を実行中

Configuration error:
設定ファイルに文法エラーが見つかりました: invalid syntax (conf.py, line 63)

make: *** [html] Error 2

Environment

• Python 3.9.4
• Sphinx 4.3.1

Problem: We cannot use async as parameter of add_js_file in Python 3.7 or later.
https://docs.python.org/3/whatsnew/3.7.html#summary-release-highlights

async and await are now reserved keywords.

Describe the solution you'd like
Proposal: add a parameter to add_js_file to specify async or not.
add_js_file(filename: str, priority: int = 500, load_async: bool = False, **kwargs: Any)

app.add_js_file('example.js', load_async=False)
# => <script src="_static/example.js"></script>

app.add_js_file('example.js', load_async=True)
# => <script src="_static/example.js" async></script>

Describe alternatives you've considered
If my proposal is not implemented, I would like to update the example of the document https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_js_file.

Options:

1. Show Python version limitation

app.add_js_file('example.js', async="async")  # only Python 3.6 or earlier (cause SyntaxError in Python 3.7 or later)
# => <script src="_static/example.js" async="async"></script>

2. Remove async="async" example
   In 2022, Python 3.6 will no longer be supported.
   ref: https://www.python.org/dev/peps/pep-0494/#lifespan
   The example causes SyntaxError in Python 3.7 and later, so It seems better to remove it than to keep it in the document.

Additional context
N/A

[tk0miya]

OMG, it must be a bug of our document. Thank you for reporting.

There are two kinds of loading methods; async and defer. So it would be better to be able to specify one of them as a value instead of parameter name (ex. loading_method="async"). I'll consider what better name is.