Adding django CMS 3 compatibility

[Bernardvdv]

Adding django CMS 3 compatibility.

Fixes #49

[Bernardvdv]

Updated test requirements

[Aiky30]

Requires the following changes to work (I have tested them):

In compat.py

The cms 3 compat is only looking for 3.6 and not less than 4:

From:
CMS_36 = LooseVersion(CMS_VERSION) < LooseVersion("3.6")

To:
CMS_LT_4 = LooseVersion(CMS_VERSION) < LooseVersion("4.0")

The tests for skipping using CMS_36, will need to reference CMS_LT_4

In tox.ini

The tox.ini file needs to have the following spec:
py{36,37,38}-dj{111,20,21,22}-sqlite-cms40-{default,versioning}
py{36,37,38}-dj22-sqlite-{cms37,cms38}-default

Notice that the py35 is missing and 38 has been added. The base list in the tox.ini will need to match this configuration too,

[Bernardvdv]

Requested changes made. Additionally, updated all import occurrences of the CMS_LT_4.

[Aiky30]

looks good to me. :-)

[Aiky30]

Damn we've missed a couple of things

1. we haven't told tox what cms37, cms38 are.
2. There's also the circle ci config to change too which is completely separate. I'll do that in a separate PR which basically removes Python 3.5 and adds 3.8 and 3.9 support.

This is pending further changes from me I'm afraid. Sorry I was hoping to bring this change in for you. :-(