bpo-45593: make SpooledTemporaryFile.truncate return new file size and improve documentation

[rooterkyberian]

https://bugs.python.org/issue45593

Currently SpooledTemporaryFile.truncate doesn't return anything.
This seems to be an old issue common for some SpooledTemporaryFile methods; most recently .seek(...) has been fixed in similar fashion.

https://bugs.python.org/issue45593

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[MaxwellDupre]

Ran 99 tests in 1.041s

OK (skipped=2)

== Tests result: SUCCESS ==
Looks ok to me.

[furkanonder]

LGTM. @rooterkyberian Can you resolve the conflict?

[rooterkyberian]

@furkanonder done - rebased to current main and resolved conflicts

[methane]

If we add 0 here, we should add 12 after fp.write(b'Hello world!') too.

[methane]

And later,

    >>> with tempfile.TemporaryFile() as fp:
    ...     fp.write(b'Hello world!')
    ...     fp.seek(0)
    ...     fp.read()
    b'Hello world!'

This output should be:

12
0
b'Hello world!'

[methane]

It seems this file is not tested by doctest.

Please leave it. Otherwise, we need to fix all output and remove all comments in sample code.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #32633580 | 📁 Comparing 1c40e9f against main (f23a183)

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3 files changed

± c-api/slots.html
± library/tempfile.html
± whatsnew/changelog.html