Drop python 2 support

[adi-]

Django 2.0 is removing Python 2.x support, so as we should do...

[ezarowny]

From #105:

For uploads, I noticed that using django-markdownx with something like django-storages doesn't work because the image stream position is set to the end of the steam when the InMemoryUploadedFile is instantiated. Simply resetting the image stream position back to the beginning should do the trick.

In the future, when you guys drop Python 2 support, something like getbuffer or getvalue could be used to get the length of the buffer instead of using tell.

[adi-]

Already in master