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

[outis]

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.