Add constraints_txt argument.

[ryanli]

This fixes #281.

[thirtyseven]

Thanks for this PR! We've been testing this patch in our repo and noticed a bash 3.x compatibility issue with the array expansion approach.

macOS ships with bash 3.2, and with set -u, "${empty_array[@]}" triggers an "unbound variable" error on empty arrays. A fix is to replace the array with ${CONSTRAINTS_TXT:+--constraint="$CONSTRAINTS_TXT"} directly, which avoids the array entirely and works on bash 3.x:

CONSTRAINTS_TXT="{{constraints_txt}}"

{{uv}} pip compile \
    {{args}} \
    ${CONSTRAINTS_TXT:+--constraint="$CONSTRAINTS_TXT"} \
    --output-file="$REQUIREMENTS_TXT" \
    "$REQUIREMENTS_IN" \
    "$@"

This applies to both pip_compile.sh and pip_compile_test.sh.

[ryanli]

Ah bash 3.2 ;-)

I've updated the commit, thanks!