Django-mjml

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* Import `requests` only if it's really needed

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* Added supporting Django v3.1

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* Added `requests` in extras require in `setup.py`
* Added MJML 4.6.3 in tests
* Upgraded MJML to 4.6.3 in dockerfile
* Updated docs

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* Added supporting Django v3.0
* Added supporting render http-server (including official MJML API https://mjml.io/api)
* Added Python 3.8 in tests
* Added MJML 4.5.1 in tests
* Upgraded MJML to 4.5.1 in dockerfile
* Upgraded Node to v12 for tcp-server
* Reorganized tests
* Updated docs

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* Fixed a trouble with unicode
* Added MJML 4.4.0 in tests
* Upgraded MJML to 4.4.0 in dockerfile

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* Removed MJML 4.0.5, 4.1.2 and 4.2.1 from tests
* Added MJML 4.3.1 in tests
* Updated tcp-server adding cleanly termination
* Upgraded MJML to 4.3.1 in dockerfile
* Updated dockerfile by using `exec`
* Added supporting Django v2.2