gh-130317: fix test_pack_unpack_roundtrip() and add docs

[skirpichev]

• skip sNaN's testing in 32-bit mode
• drop float_set_snan() helper
• use memcpy() workaround for sNaN's in PyFloat_Unpack4()
• document, that sNaN's may not be preserved by PyFloat_Pack/Unpack API

• Issue: struct (un)packing of half-precision nan floats is non-invertible #130317

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📚 Documentation preview 📚: https://cpython-previews--133204.org.readthedocs.build/

[vstinner]

!buildbot x86 Debian

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit ceff73a 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F133204%2Fmerge

The command will test the builders whose names match following regular expression: x86 Debian

The builders matched are:

• x86 Debian Non-Debug with X PR
• x86 Debian Installed with X PR

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 RHEL8 LTO 3.x (tier-1) has failed when building commit ad2f088.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/64/builds/9303) and take a look at the build logs.
4. Check if the failure is related to this commit (ad2f088) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/64/builds/9303

Failed tests:

• test_frame

Summary of the results of the build (if available):

==

Click to see traceback logs

remote: Enumerating objects: 13, done.        
remote: Counting objects:   8% (1/12)        
remote: Counting objects:  16% (2/12)        
remote: Counting objects:  25% (3/12)        
remote: Counting objects:  33% (4/12)        
remote: Counting objects:  41% (5/12)        
remote: Counting objects:  50% (6/12)        
remote: Counting objects:  58% (7/12)        
remote: Counting objects:  66% (8/12)        
remote: Counting objects:  75% (9/12)        
remote: Counting objects:  83% (10/12)        
remote: Counting objects:  91% (11/12)        
remote: Counting objects: 100% (12/12)        
remote: Counting objects: 100% (12/12), done.        
remote: Compressing objects:  20% (1/5)        
remote: Compressing objects:  40% (2/5)        
remote: Compressing objects:  60% (3/5)        
remote: Compressing objects:  80% (4/5)        
remote: Compressing objects: 100% (5/5)        
remote: Compressing objects: 100% (5/5), done.        
remote: Total 13 (delta 7), reused 7 (delta 7), pack-reused 1 (from 1)        
From https://github.com/python/cpython
 * branch                    main       -> FETCH_HEAD
Note: switching to 'ad2f0884b16e6af4087ba078d2255d4c81ae8e96'.

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HEAD is now at ad2f0884b16 gh-130317: Fix test_pack_unpack_roundtrip() and add docs (#133204)
Switched to and reset branch 'main'

configure: WARNING: no system libmpdecimal found; falling back to bundled libmpdecimal (deprecated and scheduled for removal in Python 3.15)

make: *** [Makefile:2453: buildbottest] Error 2