Py-tsdata

Fixes bug with global scope messing up the loaded data.

This should now work for most users. :)

Fixed data files not appearing in PyPI package.

This is the first actual data added to the package, taken from the excellent online textbook [Forecasting: Principles and Practice, 3rd ed](https://otexts.com/fpp3/) and the accompanying R packages [FPP3](https://github.com/robjhyndman/fpp3-package) and [tsibbledata](https://github.com/tidyverts/tsibbledata). This is meant to be a Python equivalent for students to use.

Changed the name of the package to `py-tsdata` to avoid the conflict on PyPI.