Pyfu-usb

- Add Python 3.8 support back - it did not need to be dropped in version 2.0.0.
- `importlib.metadata` is available in the standard library for Python 3.8 and
higher, so it does not need to be included as a dependency in `setup.py`.

- Drop support for Python 3.7 and 3.8.
- Migrate from `pkg_resources` to `importlib_metadata`.
- Rework `_dfuse_download` slightly: Instead of always clearing the status
before starting the download, only do it if a pipe error is detected. In newer
STM32 devices, clearing the status seems to make the first download fail.

- Ignore all `USBError` exceptions when exiting DFU mode.

- Set `long_description` of package to contents of `README.md`.

- Initial release.