annotation on h.readthedocs.io fails in via

### Steps to reproduce
1. https://via.hypothes.is/http://h.readthedocs.io/en/latest/api/#annotations:t4wXRLWaEea1XdeEWgJwow

### Expected behaviour
The page loads, the sidebar loads, the annotation focuses.

### Actual behaviour
Error downloading http://h.readthedocs.io/en/latest/api/hypothesis.yaml Failed to fetch

Note: we need labels for issues that would formerly have been filed in other repos, especially: via, h, client, wp-hypothesis, pdf.js-hypothes.is. Done, thx @ajpeddakotla.

Reproduce the failure using the via.hypothes.is URL in the issue and observe the request for hypothesis.yaml. Trace why that resource fails to load, then verify that the page and sidebar load and the annotation focuses successfully.

Written by the indexing model from the issue text.