The asyncio REPL reaches for `PYTHONSTARTUP` in isolated mode

[bswck]

Bug report

Bug description:

According to the documentation, all PYTHON* environment variables are ignored when -I is passed as a command-line option to Python.

Let's make the asyncio REPL comply with that by checking sys.flags.isolated before PYTHONSTARTUP.

#140288 rearranges and solidifies startup tests for any REPLs. It will help in testing this. Let's merge that PR first, and I'll submit a patch for this one shortly after.

Reproduction:

$ echo 'print("you should not be seeing this!")' > t.py

$ PYTHONSTARTUP=t.py ./python -Im asyncio
asyncio REPL 3.15.0a1+ (heads/main:06c779474c7, Oct 27 2025, 02:33:08) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
you should not be seeing this!
>>> import asyncio
>>>

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

[bswck]

@ZeroIntensity (mentorship) labels: stdlib, topic-asyncio, topic-repl, 3.13, 3.14.

[ZeroIntensity]

Do you want the 3.15 label too?

[bswck]

Do you want the 3.15 label too?

Yes! Missed that. Thanks :-)