"ValueError: I/O operation on closed file." when debugging from the command line

# Summary
When trying to debug pdftocio (or pdftocio) using pdb and reading the TOC from a file, pdb crashes with "ValueError: I/O operation on closed file".

# Steps to reproduce
1. create a TOC file "test.toc" (contents don't matter) and have a PDF
2. run: `python -m pdb ${bindir}/pdftocio -t test.toc in.pdf`
3. from pdb, type `c`

# Cause
By default, the toc file is opened from `sys.stdin.buffer`. If a file is set with `-t`, stdin gets closed. pdb then tries to read from stdin to get the next command, causing the exception.

Start by reproducing the reported command with Python's pdb and the pdftocio -t option. Then inspect the command-line path that opens the TOC from stdin and compare it with pdb's command input. Done means debugging can continue after entering c without a closed-file exception when -t test.toc is used.

Written by the indexing model from the issue text.