Sphinx-jinja

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Release the dependency to >4.2 to be more compatible as a deep dependency

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Readme fix

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Release the docutil dependency constraint to allow more recent version.

**Breaking change**: In order to be compatible with modern sphinx, the plugin is now in its own
module, and not anymore in the "sphinxcontrib" namespace.

You will need to change you conf.py with the following pseudo patch:


- extensions = ['sphinxcontrib.jinja']
+ extensions = ['sphinx_jinja']

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Implement the header_update_levels flag, which allows jinja blocks to use the same header (titles) level as the parent block.

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Use app.srcdir as jinja_base.

This affects people that include files from jinja context and use separate
source and build directories. They should change:


.. jinja::
:file: source/test.rst


to


.. jinja::
:file: test.rst


* Add `header_update_levels` option to let headers behave in the same way as
if include via the `include` directive.

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untight the dependency on jinja2 from ^2.11 to >=2.11