Run with Python 3; no packages required. Amounts are integer cents.

From this directory (macOS/Linux shell):
INVOICE_MODULE=invoice_buggy python3 -m unittest -v test_invoice
INVOICE_MODULE=invoice_fixed python3 -m unittest -v test_invoice

PowerShell:
$env:INVOICE_MODULE = "invoice_buggy"
python -m unittest -v test_invoice
$env:INVOICE_MODULE = "invoice_fixed"
python -m unittest -v test_invoice

The buggy run intentionally exits 1 with one assertion failure.
The fixed run exits 0 with all three tests passing.
This constructed example tests arithmetic on integer-cent line items only.