Changes for users: none.

Notes:

- This adds CI testing with lowest and highest Python versions we
  support.
- The main motivation for this is that we have Windows-specific code I'm
  worried I might break with improvements, like improvements in `dotenv
  run` error handling (coming soon).
- I went for the least intrusive changes for now, and disabled tests
  which would fail unless they were trivial to adjust.
  - We have tests using `sh` (Unix-only module) which should be possible
    to fix later. Those tests are disabled on Windows.
  - Also tests relying on the fact that environment variables are case
    sensitive, which isn't the case on Windows. This is going to be more
    tricky to fix. Those tests are also disabled on Windows.
- To check for the platform, I used `sys.platform == "win32"`
  everywhere, which seems to be the best practice.