Pdb: inverted PYTHON_BASIC_REPL check in `_pyrepl_available()`

# Bug report

### Bug description:

The condition `if not os.getenv("PYTHON_BASIC_REPL")` is inverted, causing _pyrepl to be enabled when the env var is set and disabled when it's not set.

### CPython versions tested on:

CPython main branch

### Operating systems tested on:

Windows


### Linked PRs
* gh-149174