remove limitation: "Altering a model field from or to AutoField at migration"

[n2ygk]

First, let me say a huge thank you for picking up this project!
I haven't had a chance to look at this fork in any detail yet, so please forgive any duplication. I did search issues and this might overlap with #38 and #39.

I was wondering if you had looked at michiya#189 which I submitted back in November 2018 that removes the "Altering a model field from or to AutoField at migration" limitation.

[OskarPersson]

I haven't looked into michiya#189 in detail. But if you add a PR here that either has some tests added to the repo that gets fixed by it or that fixes some tests in the Django test suite, I would gladly take a look at it.

[n2ygk]

I'll add this to my backlog. No promises just yet as we've actually decided to drop use of SQL Server since it lacks official Django support and MSFT hasn't stepped up despite attempts so far to get them to do so: michiya#190