The current version is broken on the version of mautrix-python in nixpkgs and there are a couple fixes scattered in unreleased commits. Simply update to HEAD to get it working again.
It appears that these are bumped to match manual testing however they do not work well in nixpkgs as every time a dependency is updated this package breaks. To resolve this issue just strip the max version requirements and assume that it works.
Ideally we would do a semver-type comparison but this is likely not an issue because a new major version would need a new attribute name anyways.
Kevin Cox