Use txmongo's find_with_cursor method

[lafrech]

This PR fixes a deprecation warning due to API changes in txmongo.

I had to bump minimal txmongo version to 19.2.0 (latest) for the tests to pass. I think we should be able to support older versions. I suspect I just didn't find the proper pymongo, twmongo and perhaps twisted and pytest-twisted combination. The incomplete changelog didn't help. I went through the commits in txmongo and couldn't find the reason versions prior to 19.2.0 would fail.

I don't mind bumping this since it is only a test dependency, and we're releasing a major version, but for the sake of understanding and because it would be nice to know which txmongo versions are supported, explanation welcome.

BTW, @wfscheper, I hope you're doing well and I'd be glad to get some feedback on umongo 3 beta if you can find the time to give it a try.

[coveralls]

Coverage increased (+0.1%) to 95.766% when pulling 4deaa7b on update_txmongo into d9ca100 on master.

[lafrech]

I forgot to bump txmongo version in setup.py.
Should be at least 16.3.0, maybe more (see above).

[lafrech]

Actually, after checking the commit history once again:

• 16.3.0 for this change to work (we use methods added in 16.3.0)
• 18.2.0 to support pymongo 3.7, which is our minimum required version
• 19.2.0 to support pymongo 3.9 and perhaps MongoBD 4 issues

I guess my test issues came from using MongoDB 4.

I don't mind requiring 19.2.0 for a new major version even if it is not actually needed (people could use pymongo 3.7 / Mongo 3.x with txmongo 18.2.0).

[lafrech]

Let's move on with this.