Add py.typed

[davetapley]

I just bumped to 1.0.5 to get #103.
It works great but VSCode still says:

---

I didn't realize you need a py.typed file: https://peps.python.org/pep-0561/#packaging-type-information

Good with a PR?

I have a couple of other hints too.

[thodson-usgs]

great, @davetapley. Please, go ahead.

[davetapley]

This surfaced again for me as:

error: Stub file not found for "dataretrieval.nwis" (reportMissingTypeStubs)

To avoid confusion with master -> main and #184 I'll open a new PR...