Doker

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Changed:
~~~~~~~~

* Use latest ``reportlab`` instead of previously fixed v3.5.18
* Local copy of ``rst2pdf`` updated to v0.96 and patched
* Moved onto the Python 3 with no regret

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Fixed:
~~~~~~

* Set ``reportlab`` version to v3.2.18 due to https://github.com/rst2pdf/rst2pdf/issues/773 and https://github.com/rst2pdf/rst2pdf/issues/772
* Added ``six`` package to the dependencies list
* Windows: Fixed "Permission denied" error while opening temp files
* Windows: Added workaround for Jijnja2 finds template file by absolute path

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Fixed:
~~~~~~

* YAML loading is safe now (see https://msg.pyyaml.org/load)
* Generated stylesheet JSON files became temporary