This commit has the following benefits:

- Remove `sh` as a development dependency.
- Increase test coverage on Windows.
- Improve the robustness of some tests against leftover `.env` files in
  the repository.
    - This is not perfect yet: If you have a `.env` file in your
      repository, it still disrupts some tests (for `find_dotenv`).
- Improve the readability of error messages for some tests.