In pip-2.0 signature of unpack_url changed

[Jacob Rief]

In https://github.com/sarugaku/pip-shims/blob/master/src/pip_shims/compat.py#L769
unpack_fn points onto pip's function unpack_url, which changed its signature between version 20.2.4 and 20.3, resulting in a error:

  File "/usr/local/lib/python3.8/site-packages/pip_shims/compat.py", line 769, in shim_unpack
     return unpack_fn(**unpack_kwargs)  # type: ignore
 TypeError: unpack_url() missing 1 required positional argument: 'download'