RF: Use optional_package to allow code to assume pyzstd is present

[effigies]

Where possible, we try to code as if optional dependencies are present. optional_package provides three things:

1. The requested module, or a fake module that raises an error when you try to access its namespace. If we can use only this, things are cleaner.
2. A boolean indicating whether the module could be loaded. In cases where you really need to know, you can use this.
3. A setup_module function that will instruct test runners to skip an entire test file if it could not be loaded. Mainly useful for things like minc2 which can't be opened at all without h5py.

So this is a PR to your PR that tries to use that approach, as well as adds pyzstd to the optional dependencies we test on.

[effigies]

Probably a good idea to just cherry-pick a1b1172 for now. Getting failures here: https://github.com/effigies/nibabel/actions/runs/890416068

Fixed.