Django-mjml

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* Added `MJML_CHECK_CMD_ON_STARTUP` setting (thanks to Marcel Chastain)
* Added Python 3.7 in tests
* Added MJML v.4.2.1 in tests
* Removed MJML v.2.3.3 from tests
* Updated MJML to 4.2.1 in dockerfile

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* Fixed Popen PIPE subprocess deadlock by using TemporaryFile for stdout

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* Added supporting MJML v4.1.2
* Added supporting Django v2.1

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* Added supporting MJML v4.1.0
* Added .pyup.yaml
* Updated tests
* Added dockerfile for tcpserver
* Remove mjml 3.0.2, 3.1.1 and 3.2.2 from tests

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* Add stopping tcpserver on SIGINT

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* Add support MJML v4
* Tcpserver doesn't skip mjml errors now (thanks yourcelf)
* Refactor arguments in tcpserver
* Fix incomplete sending data via socket (thanks cavanierc)