Pin sphinxcontrib-httpexample==0.7.0 in readthedocs-requirements.txt:

This is to avoid an issue in version 0.10.0 that prevents
requests/responses being included from the filesystem, e.g. using
the :requests: option.

In the medium / long term, we should
- update sphinxcontrib-httpexample to 0.9.1 (which still works)
- clean up the requirements in plone.restapi (duplication between
  requirements.txt and docs/source/readthedocs-requirements.txt)
- switch the ReadTheJobs build to use the same requirements that
  are used for local development / docs builds.

docs/source/readthedocs-requirements.txt

@@ -1,2 +1,2 @@
 sphinxcontrib-httpdomain
-sphinxcontrib-httpexample
+sphinxcontrib-httpexample==0.7.0