Fix downloading remote datasets to cache on Windows

[hakonanes]

Description of the change

Fixes #654 reported by @ericpre.

We use Pooch to manage our kikuchipy.data module. Pooch requires file paths with unix-style separators "/" only. However, os.path.join(), which returns paths with "\"-separators on Windows, was used to create paths passed to Pooch in one place in our data module. Thus the reported error.

This PR should fix this issue by using pathlib.Path to create paths for Pooch.

I tested that it works on a local Windows machine. This issue was not caught by our test suite because it is too computationally expensive to download a file from Zenodo in a test. I'm not sure how to make a computationally inexpensive test that would catch this without calling internal methods in Pooch.

Progress of the PR

• [n/a] Docstrings for all functions
• [n/a] Unit tests with pytest for all lines
• [x] Clean code style by running black via pre-commit

Minimal example of the bug fix or new feature

The following works on Windows:

>>> import kikuchipy as kp
>>> s = kp.data.si_wafer(allow_download=True)

For reviewers

• [ ] The PR title is short, concise, and will make sense 1 year later.
• [ ] New functions are imported in corresponding __init__.py.
• [ ] New features, API changes, and deprecations are mentioned in the unreleased section in CHANGELOG.rst.
• [ ] New contributors are added to release.py, .zenodo.json and .all-contributorsrc with the table regenerated.