MRG: fix deprecated method warning in lib.rs

[ctb]

Fix this warning:

warning: use of deprecated method `pyo3::deprecations::GilRefs::<T>::function_arg`: use `&Bound<'_, T>` instead for this function argument
   --> src/lib.rs:310:45
    |
310 | fn sourmash_plugin_branchwater(_py: Python, m: &PyModule) -> PyResult<(...
    |                                             ^
    |
    = note: `#[warn(deprecated)]` on by default

by swiping code from directsketch.

Note, it looks to me like this will make the pyo3 version bump in #382 work as well 🎉

[ctb]

ready for review & merge @bluegenes !!

[bluegenes]

lgtm!