Drop support for Django 1.6 and 1.7, Python 3.3

[sjkingo]

As of December 2015, 1.8 is the LTS and 1.6 and 1.7 are unsupported.

Is there any point continuing to officially support 1.6 and 1.7? We're not going to intentionally break things, but if there is a Django feature that is in >1.8 only, we shouldn't shy away from using it.

[sjkingo]

Also Python 3.3 is unsupported upstream so we need to remove that and support 3.5 instead.

I'm not yet happy with dropping 2.7 support in case someone is using it still.

[sjkingo]

3.5 works out of the box

[hugovk]

Here's the pip installs for django_auth_ldap3 from PyPI for the last year -- no 3.3!

pypinfo -d 365 --percent --pip django_auth_ldap3 pyversion
python_version percent download_count
-------------- ------- --------------
2.7              58.6%          1,445
3.6              18.2%            448
3.5              15.5%            382
3.4               7.7%            191

[frennkie]

I like that this uses ldap3 and there does not require LDAP/SASL libs and so on.
But how about current Python and Django support? Is it tested on Django 1.11 and 2.0.x?