Support for text/x-amp-html mime type

[SergeNasr]

Issue Summary - Feature Request

I am trying to use the SendGrid python client to send AMP dynamic emails. AMP emails require the text/x-amp-html content type as per SendGrid's doc
However the python library does not support the text/x-amp-html mime type yet.

[thinkingserious]

Hello @SergeNasr,

Thank you for this request! This issue has been added to our internal backlog to be prioritized. Pull requests and +1s on the issue summary will help it move up the backlog.

Note: the mime-type ordering will likely need to be considered.

With best regards,

Elmer

[childish-sambino]

May be fixed by #851

[SergeNasr]

@childish-sambino I think we would also need to add a if amp_content in https://github.com/sendgrid/sendgrid-python/blob/v6.1.0/sendgrid/helpers/mail/mail.py#L74 and also the mime-type ordering linked by @thinkingserious

[arun-tvs-sage]

@SergeNasr have added support for this here. Feel free to clone that branch and use if you are blocked on something and add your feedback. Thanks