Arrow key history broken when stdin is passed explicitly on pdb

[zelosleone]

Bug report

Bug description:

Description:
When using pdb through pytest (or any tool that passes stdin explicitly), arrow key history doesn't work even when stdin is a real TTY. And currently, we assume readline.backend exists even on windows where pyreadline3 doesn't have readline.backend
astral-sh/uv#17174 directly related to this.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-143082: Fix readline backend check and stdin isatty condition in cmd and pdb modules #143083