refactor: make XModuleMixin extend XBlockMixin, not XBlock

In order to get typechecking to pass in XBlock, we needed to make
ScorableXBlockMixin a subclass of XBlockMixin. This change, while
sensible, creates an ambiguous method resolution order in the
ProblemBlock, stemming from the fact that XModuleMixin inherits directly
from XBlock.

The fix is to simply make XModuleMixin inherit from XBlockMixin
instead-- this makes sense because XModuleMixin is a helper mixin, not
an XBlock itself. We then must add XBlock as an explicit subclass to all
blocks which inherit from XBlock. This should have no functional impact,
but *should* help us add type hints to the xmodule/ folder in the
future.

Using ProblemBlock as an example... before (XBlock < 6.0.0) we have:

  ProblemBlock <- ScorableXBlockMixin
               <- XModuleMixin <- XBlock <- Blocklike

after (XBlock >= 6.0.0) we have:

  ProblemBlock <- ScorableXBlockMixin <- XBlockMixin <- Blocklike
               <- XModuleMixin        <- XBlockMixin <- Blocklike
               <- XBlock                             <- Blocklike

Author: kdmccormick

xmodule/annotatable_block.py

@@ -35,6 +35,7 @@ class AnnotatableBlock(
     XModuleToXBlockMixin,
     ResourceTemplates,
     XModuleMixin,
+    XBlock,
 ):
     """
     Annotatable XBlock.

xmodule/capa_block.py

@@ -142,6 +142,7 @@ class ProblemBlock(
     XModuleToXBlockMixin,
     ResourceTemplates,
     XModuleMixin,
+    XBlock,
 ):
     """
     An XBlock representing a "problem".

xmodule/conditional_block.py

@@ -45,6 +45,7 @@ class ConditionalBlock(
     ResourceTemplates,
     XModuleMixin,
     StudioEditableBlock,
+    XBlock,
 ):
     """
     Blocks child blocks from showing unless certain conditions are met.

xmodule/error_block.py

@@ -48,6 +48,7 @@ class ErrorBlock(
     XModuleToXBlockMixin,
     ResourceTemplates,
     XModuleMixin,
+    XBlock,
 ):  # pylint: disable=abstract-method
     """
     Block that gets shown to staff when there has been an error while

xmodule/hidden_block.py

@@ -18,6 +18,7 @@ class HiddenBlock(
     XmlMixin,
     XModuleToXBlockMixin,
     XModuleMixin,
+    XBlock,
 ):
     """
     XBlock class loaded by the runtime when another XBlock type has been disabled

xmodule/html_block.py

@@ -353,7 +353,7 @@ def index_dictionary(self):
 
 
 @edxnotes
-class HtmlBlock(HtmlBlockMixin):  # lint-amnesty, pylint: disable=abstract-method
+class HtmlBlock(HtmlBlockMixin, XBlock):  # lint-amnesty, pylint: disable=abstract-method
     """
     This is the actual HTML XBlock.
     Nothing extra is required; this is just a wrapper to include edxnotes support.
@@ -374,7 +374,7 @@ class AboutFields:  # lint-amnesty, pylint: disable=missing-class-docstring
 
 
 @XBlock.tag("detached")
-class AboutBlock(AboutFields, HtmlBlockMixin):  # lint-amnesty, pylint: disable=abstract-method
+class AboutBlock(AboutFields, HtmlBlockMixin, XBlock):  # lint-amnesty, pylint: disable=abstract-method
     """
     These pieces of course content are treated as HtmlBlocks but we need to overload where the templates are located
     in order to be able to create new ones
@@ -409,7 +409,7 @@ class StaticTabFields:
 
 
 @XBlock.tag("detached")
-class StaticTabBlock(StaticTabFields, HtmlBlockMixin):  # lint-amnesty, pylint: disable=abstract-method
+class StaticTabBlock(StaticTabFields, HtmlBlockMixin, XBlock):  # lint-amnesty, pylint: disable=abstract-method
     """
     These pieces of course content are treated as HtmlBlocks but we need to overload where the templates are located
     in order to be able to create new ones
@@ -435,7 +435,7 @@ class CourseInfoFields:
 
 @XBlock.tag("detached")
 @XBlock.needs('replace_urls')
-class CourseInfoBlock(CourseInfoFields, HtmlBlockMixin):  # lint-amnesty, pylint: disable=abstract-method
+class CourseInfoBlock(CourseInfoFields, HtmlBlockMixin, XBlock):  # lint-amnesty, pylint: disable=abstract-method
     """
     These pieces of course content are treated as HtmlBlock but we need to overload where the templates are located
     in order to be able to create new ones

xmodule/library_content_block.py

@@ -85,6 +85,7 @@ class LibraryContentBlock(
     ResourceTemplates,
     XModuleMixin,
     StudioEditableBlock,
+    XBlock,
 ):
     """
     An XBlock whose children are chosen dynamically from a content library.

xmodule/lti_block.py

@@ -284,6 +284,7 @@ class LTIBlock(
     XModuleToXBlockMixin,
     ResourceTemplates,
     XModuleMixin,
+    XBlock,
 ):  # pylint: disable=abstract-method
     """
     THIS MODULE IS DEPRECATED IN FAVOR OF https://github.com/openedx/xblock-lti-consumer

xmodule/poll_block.py

@@ -42,6 +42,7 @@ class PollBlock(
     XModuleToXBlockMixin,
     ResourceTemplates,
     XModuleMixin,
+    XBlock,
 ):  # pylint: disable=abstract-method
     """Poll Block"""
     # Name of poll to use in links to this poll

xmodule/randomize_block.py

@@ -6,6 +6,7 @@
 from django.utils.functional import cached_property
 from lxml import etree
 from web_fragments.fragment import Fragment
+from xblock.core import XBlock
 from xblock.fields import Integer, Scope
 from xmodule.mako_block import MakoTemplateBlockBase
 from xmodule.seq_block import SequenceMixin
@@ -27,6 +28,7 @@ class RandomizeBlock(
     XModuleToXBlockMixin,
     ResourceTemplates,
     XModuleMixin,
+    XBlock,
 ):
     """
     Chooses a random child xblock. Chooses the same one every time for each student.