FileCheck.py exits with 141 when run by LIT (all checks are checked but there is still some input in stdin)

I can reproduce this behavior on a simple Python script when the `set -o pipefail` command is set.

To clarify this further I have asked about this on StackOverflow:

[Getting exit code 141 when reading from stdin with a Python script with “set -o pipefail” set](https://stackoverflow.com/questions/59436858/getting-exit-code-141-when-reading-from-stdin-with-a-python-script-with-set-o).