Drop support for Python 3.6, enable support for Markdown 3.4

[tomaszn]

There are incompatibilities with Markdown 3.4.

@agusmakmun Can you take a look at these?
https://github.com/Python-Markdown/markdown/blob/master/docs/change_log/release-3.4.md

I suggest releasing this as v1.7.0 (bump the minor version).

[agusmakmun]

hello @tomaszn, thank you for making this changes.. can we somehow support the python 3.6 as well?
let say, for python 3.6 will be use Markdown < 3.4, then for the higher version will use Markdown < 3.5

[DerDennisOP]

You need to remove the "md_globals" input parameter from every Extension Class.
Also make sure you use the .register function:
md.inlinePatterns["autolink"] = UrlizePattern(URLIZE_RE, md) -> md.inlinePatterns.register((URLIZE_RE, md), 'autolink', 100)

[DerDennisOP]

Also Python 3.6 is depricated now for almost 5 years and security support ended 1 year ago. I don't think we need Python 3.6 support

[agusmakmun]

Hello @tomaszn sorry for closing this PR, this fixes is alraedy in this commit:

82f9dad

https://github.com/agusmakmun/django-markdown-editor/releases/tag/v1.6.43