There are currently two multipart implementations on PyPI: multipart and python-multipart. Both install the same multipart package, and therefore cannot be installed simultaneously.

Both starlette and synapse use (apparently more popular) python-multipart. This is probably understandable, given that prior to June, the last release of multipart was in 2021. However, treq now started using multipart, which means it is no longer possible to install treq and starlette on the same system.

Please consider using python-multipart for the sake of compatibility instead.