Make database backend __init__() methods friendlier to subclassing

[Chris Jerdonek]

The __init__() method for most or all of the database backends looks something like the following:

def __init__(self, *args, **kwargs):
    super(DatabaseWrapper, self).__init__(*args, **kwargs)

    self.features = DatabaseFeatures(self)
    self.ops = DatabaseOperations(self)
    self.client = DatabaseClient(self)
    self.creation = DatabaseCreation(self)
    self.introspection = DatabaseIntrospection(self)
    self.validation = DatabaseValidation(self)

(​Here is a link to this code for the MySQL backend.)

Notice that each of the database classes is "hard-coded."

Since much of the behavior of the backends is defined by these classes (and so can be modified by changing these classes), it would be nice if it were easier to change what class is used. I believe this could be done simply by making each of these classes a class attribute. This pattern is used to good effect, for example, throughout the test framework. You can see this being done in the definition of DiscoverRunner, for example, ​here:

class DiscoverRunner(object):

    test_suite = unittest.TestSuite
    parallel_test_suite = ParallelTestSuite
    test_runner = unittest.TextTestRunner
    test_loader = unittest.defaultTestLoader
    reorder_by = (TestCase, SimpleTestCase)

[Chris Jerdonek]

I started working on this in ​this PR.

[Tim Graham <timograham@…>]

In 7ca3b391:

Fixed #27170 -- Added DatabaseWrapper class attributes to ease subclassing.