Make finalizeSchema more robust by not handling behavior schema classes.

Backported from version 2.0.

This fixes test failures on Plone 5.2 Python 3 when used together with plone/plone.dexterity#189, which is a backport of a memory leak fix.
Failures are like this:

```
File "/home/jenkins/.buildout/eggs/cp38/plone.autoform-1.9.1-py3.8.egg/plone/autoform/tests/../autoform.rst", line 50, in autoform.rst
Failed example:
    xmlconfig.xmlconfig(StringIO(configuration))
Exception raised:
    Traceback (most recent call last):
      File "/srv/python3.8/lib/python3.8/doctest.py", line 1336, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest autoform.rst[11]>", line 1, in <module>
        xmlconfig.xmlconfig(StringIO(configuration))
      File "/home/jenkins/.buildout/eggs/cp38/zope.configuration-4.4.1-py3.8.egg/zope/configuration/xmlconfig.py", line 732, in xmlconfig
        context.execute_actions(testing=testing)
      File "/home/jenkins/.buildout/eggs/cp38/zope.configuration-4.4.1-py3.8.egg/zope/configuration/config.py", line 799, in execute_actions
        reraise(
      File "/home/jenkins/.buildout/eggs/cp38/zope.configuration-4.4.1-py3.8.egg/zope/configuration/_compat.py", line 31, in reraise
        raise value.with_traceback(tb)
      File "/home/jenkins/.buildout/eggs/cp38/zope.configuration-4.4.1-py3.8.egg/zope/configuration/config.py", line 791, in execute_actions
        callable(*args, **kw)
      File "/home/jenkins/.buildout/eggs/cp38/plone.supermodel-1.6.5-py3.8.egg/plone/supermodel/model.py", line 116, in finalizeSchemas
        for schema in sorted(schemas):
    zope.configuration.config.ConfigurationExecutionError: File "/home/jenkins/.buildout/eggs/cp38/plone.supermodel-1.6.5-py3.8.egg/plone/supermodel/configure.zcml", line 9.4-12.10
          <zcml:customAction
              handler=".model.finalizeSchemas"
              order="9999999"
              />

        TypeError: '<' not supported between instances of 'Provides' and 'InterfaceClass'
```

The test failures are mostly in `plone.autoform` and in `plone.app.registry`, but strangely you do not get the error when you run only the tests for one of those packages.
You really need to run the entire Plone test suite (`bin/test` in coredev, without options).  So there is likely some test leakage.

Python 2 does not have this problem: sorting on Python 3 is handled differently.
On Python 2:

```
>>> from zope.interface.declarations import Implements, Provides
>>> sorted([Implements(), Provides(object)])
[classImplements(?), directlyProvides(object)]
```

On Python 3:

```
>>> from zope.interface.declarations import Implements, Provides
>>> sorted([Implements(), Provides(object)])
Traceback (most recent call last):
  File "<console>", line 1, in <module>
TypeError: '<' not supported between instances of 'Provides' and 'Implements'
```

news/186.feature

@@ -0,0 +1,3 @@
+Make finalizeSchema more robust by not handling behavior schema classes.
+Backported from version 2.0.
+[maurits]

plone/supermodel/model.py

@@ -107,8 +107,28 @@ def finalizeSchemas(parent=Schema):
         )
 
     def walk(schema):
-        yield schema
-        for child in schema.dependents.keys():
+        # When we have behaviors on the Plone site root we got some schemas that
+        # are not SchemaClasses.  Same is true on Plone 5.2 if we use a backported
+        # fix for plone.dexterity to use 'Provides' instead of 'Implements', which
+        # should fix a memory leak:
+        # https://github.com/plone/plone.dexterity/pull/189
+        # If we yield such a schema, then 'sorted(schemas)' below will fail on
+        # Python 3 with: TypeError: '<' not supported between instances of
+        # 'Provides' and 'InterfaceClass'.
+        if isinstance(schema, SchemaClass):
+            yield schema
+
+        # This try..except is to handle AttributeError:
+        # 'VerifyingAdapterLookup' object has no attribute 'dependents'.
+        # afaik this happens in tests only.
+        # We have issue https://github.com/plone/plone.supermodel/issues/14
+        # to find out why this is happening in the first place.
+        try:
+            children = schema.dependents.keys()
+        except AttributeError:
+            children = ()
+
+        for child in children:
             for s in walk(child):
                 yield s
 

setup.py

@@ -7,7 +7,7 @@
 def read(*rnames):
     return open(os.path.join(os.path.dirname(__file__), *rnames)).read()
 
-version = '1.6.6.dev0'
+version = '1.7.0.dev0'
 
 long_description = (
     read('README.rst') + '\n' +