`Path.cwd()` is missing a return type annotation

Mypy reports:

```
error: Call to untyped function "cwd" of "Path" in typed context [no-untyped-call]
```

–& indeed:

https://github.com/jaraco/path/blob/0f86209f11199ceaff788c73f96ea5b5ec2318ee/path/__init__.py#L254-L259

The implementation is in path/__init__.py at lines 254-259; start by reading Path.cwd() and its nearby annotations. Add the return type annotation, then run the relevant mypy check or project test suite and confirm the no-untyped-call error is gone.

Written by the indexing model from the issue text.