Webdav4

Pins httpx version to 0.20.0 till it releases 1.0 stable release.

There is an incompatible change, that the redirects are not followed by default.
This is because of the change in default in httpx itself. To follow redirects by default,
pass `follow_redirects=True` in `webdav4.fsspec.WebdavFileSystem` or in `webdav4.client.Client`.

If you cannot change the API for some reason, you can set `FSSPEC_DAV_FOLLOW_REDIRECTS=true` or
override through fsspec's config.

- Add trailing slash in url on MKCOLs

It is now compatible with `fsspec 2021.08.X` as well.

- fsspec updated to 2021.07.0.
- httpx updated to 0.18.2.
- callback support on put_file, as per 2021.07.0 requirements,
thanks to isidentical.
- `upload_fileobj` - extended API in fsspec implementations
with callback support.
- tests for `peek_filelike_length` and `upload_fileobj`.
- Packaging badges in the README, thanks to efiop.
- dev packages updated,
specifically, `Sphinx`, `sphinx-copybutton`, `furo`, `pylint` and `WsgiDAV`.

- performance improvement in fsspec new release for find
- bugfix on stream during seek, where it could use Ranged response
to compute the size of the stream (whereas it should have used size
of Initial response).
- Weekly package updates

No other changes.