- Add a couple of extra type checks that Django makes:
1) 'flat' and 'named' can't be used together.
2) 'flat' is not valid when values_list is called with more than one field.
* Support returning the correct values for the different QuerySet methods when using .values() and .values_list().
* Fix slicing on QuerySet. Fix django queries test, and remove some ignored errors that are no longer needed.
* Remove accidental change in RawQuerySet.
* Readded some still-necessary ignores to aggregation django test.
* Add more tests of first/last/earliest/last/__getitem__, per mkurnikov's comments.
- Fix .iterator()
* Re-add Iterator as base-class of QuerySet.
* Make QuerySet a Collection.
* - Fix return type for QuerySet.select_for_update().
- Use correct return type for QuerySet.dates() / QuerySet.datetimes().
- Use correct type params in return type for QuerySet.__and__ / QuerySet.__or__
- Re-add Sized as base class for QuerySet.
- Add test of .all() for all _Row types.
- Add test of .get() for all _Row types.
- Remove some redundant QuerySet method tests.
* Automatically fill in second type parameter for QuerySet.
... if second parameter is omitted.
* Fix bug where models with a class variable using a manager defined would interfere with other managers.
- Fill in the type argument for that particular instance of the manager, rather than modifying the bases of the Manager type.
- Instantiate a new Instance from determine_proper_manager_type so The code doesn't crash under mypy-mypyc.
* Use helpers.reparametrize_instance per review comment.
* Updated ignored errors in Django test for get_objects_or_404.
- For some reason, `Manager[nothing]` is now removed from expected types.
However, I think this makes sense anyway, as Manager is a subclass of QuerySet.
* Make CharField(blank=True) not be considered nullable
The documentation on [blank](https://docs.djangoproject.com/en/2.1/ref/models/fields/#blank) says that it "will allow the entry of an empty value", which for a string is just a 0-length string. This patch allows `CharField(blank=True,...)` to no longer be considered `Optional`.
closes#38
* fixed tests for `CharField(blank=True)`
* allow blank CharField to be nullable in the constructor, but the underlying type
is str (unless `null=True`)
