How to: dependency unvendoring
force use of system modules  instead of the vendored versions:

| env-var              | what it does |
| -------------------- | ------------ |
| `PRTY_SYS_ALL`       | all of the below |
| `PRTY_SYS_DNSLIB`    | replace [stolen/dnslib](./copyparty/stolen/dnslib) with [upstream](https://pypi.org/project/dnslib/) |
| `PRTY_SYS_IFADDR`    | replace [stolen/ifaddr](./copyparty/stolen/ifaddr) with [upstream](https://pypi.org/project/ifaddr/) |
| `PRTY_SYS_QRCG`      | replace [stolen/qrcodegen.py](./copyparty/stolen/qrcodegen.py) with [upstream](https://github.com/nayuki/QR-Code-generator/blob/master/python/qrcodegen.py) |

to debug, run copyparty with `PRTY_MODSPEC=1` to see where it's getting each module from