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Feature release.
* Refactor to make ``SourcesBuilder`` the default class for builds from
template and via strings.
* work round changes in Sphinx warnings for files not in toctree. By default
string-based pages are ignored for the toctree, and we do not put any pages
into the toctree by default. Use new ``toctree_pages`` class attribute to
put pages into the toctree.
* Allow master documents other than ``contents.rst`` by reading ``conf.py``
for the ``master_doc`` variable.
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