Add support for file:// URLs for intersphinx

[idclifford]

It seems that the latest version of pydoctor does not support file:// URLs for intersphinx inventory files. This can be a problem if you aren't hosting your docs online, so I would like to request that this be added. Attached is my proposed patch to sphinx.py to add file:// support of the python library requests_file is available.

sphinx.py.patch

[tristanlatr]

Hello @idclifford and thanks a lot for opening this issue and proposing a patch! It's really appreciated.

First, could you please submit your patch as a Github PR ?
Then I'll review your code :)

This issue is more or less a duplicate of #741,

Many thanks,