Improve deprecation messages for backported deprecations

[AlexWaygood]

Summary

#15750 intentionally backported several deprecations to older Python versions. But a message such as "Deprecated since Python 3.14" is now confusing if you're running a type checker with --python-version=3.13, for example -- it looks like a bug, even though it isn't. We had a user report this on Discord for ty: https://discord.com/channels/1039017663004942429/1279201882337705994/1529152755644502076.

This updates those messages to:

• remove misleading version-of-deprecation claims for these "time-travelling" deprecations
• add the Python 3.17 removal date for the deprecated tkinter.Variable trace methods;
• briefly explain the deprecation reasons for a few particularly problematic APIs, including argparse.FileType, CGIHTTPRequestHandler, importlib.resources.contents, and nturl2path.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

aiohttp (https://github.com/aio-libs/aiohttp)
- aiohttp/worker.py:253:9: error: function asyncio.events.set_event_loop_policy is deprecated: Deprecated since Python 3.14; will be removed in Python 3.16.  [deprecated]
+ aiohttp/worker.py:253:9: error: function asyncio.events.set_event_loop_policy is deprecated: Deprecated; will be removed in Python 3.16.  [deprecated]