Mjml

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- fix non-ascii content in included files on Windows

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- mj-button: prevent target="_blank" for mailto links (Thunderbird)
- fix `<mj-include>`
- update test suite to check against html generated by mjml 4.9.0 and
port required fixes to get the test suite passing.
- fix `<mj-image>` with `href` attribute

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- support `<mj-include>` for body components
- mjml_to_html(): also accept plain strings as input
- add support for json-like dict as input
- avoid deprecation warnings about invalid escape sequences in regex patterns
- add support for mj-font, mj-preview and mj-raw
- better error message for unknown tags

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- handle `<mj-head>` section containing HTML comments
- also render attributes with empty values if set explicitely
- implement (basic) support for mj-class
- port "Component.allowedAttributes" from JS mjml
- ability to render "welcome-email.mjml" from "mjmlio/email-templates"

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- stop shipping tests in wheel
- move all requirements to setup.cfg

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- mjml: always return "binary" data (UTF-8) to avoid encoding problems in Windows