gh-133879: Copyedit "What's new in Python 3.15"

[hugovk]

Fix syntax highlighting and a hyphen.

Other changes to match 3.14 layout:

• Add "New deprecations" subheading, list them as bullet points
• Put more important "Removed" before "Deprecations"
• End with "Build changes" and "Porting to..."

• Issue: What's New in Python 3.15 (copyediting) #133879

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