Re: local intersphinx

["Rebecca N. Palmer" <rebecca_palmer@zoho.com>]

Control: reassign -1 python3-sphinx-astropy
Control: tags -1 patch
(untested)

(If reading this in the bug, see 
https://lists.debian.org/debian-mentors/2020/01/msg00295.html.)

An intersphinx_mapping can specify multiple alternatives for where to 
find the inventory referred to, and these can be a mix of URLs and local 
files:

https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping

Hence, to prefer the local inventory if it is installed, but still allow 
downloading for non-buildd users:

intersphinx_mapping = {
    'python': ('https://docs.python.org/3/',
-              (None, 'http://data.astropy.org/intersphinx/python3.inv')),
+              ('/usr/share/doc/python3-doc/html/objects.inv', None, 
'http://data.astropy.org/intersphinx/python3.inv')),
[...similar for the others...]

These local inventory files are in the -doc packages, so 
(build-)dependencies on these are also required.