Synapse v1.46.0 (released on 2021-11-02) added password provider support to its new module API (see matrix-org/synapse#10548).

The old password_providers configuration (and module code) still works, but upgrading to the new API is probably better.

I've already done it for matrix-synapse-shared-secret-auth with this commit (along with other improvements, like a custom login type), so you can take inspiration from that.