Xblock

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Suppress a spurious warning when using lazily-translated text as the default
value of a String field.

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* Add capability for XBlockAsides to apply only to XBlocks that match certain conditions

1.0 - Python 3
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* Introduce Python 3 compatibility to the xblock code base.
This does not enable Python 2 codebases (like edx-platform) to load xblocks
written in Python 3, but it lays the groundwork for future migrations.

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No notes provided.

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* Separate Fragment class out into new web-fragments package

* Make Scope enums (UserScope.* and BlockScope.*) into Sentinels, rather than just ints,
so that they can have more meaningful string representations.

* Rename `export_xml` to `add_xml_to_node`, to more accurately capture the semantics.

* Allowed `Runtime` implementations to customize loading from **block_types** to
`XBlock` classes.

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* Added services available through `Runtime.service`, once XBlocks have
announced their desires with `XBlock.needs` and `XBlock.wants`.

* The "i18n" service provides a `gettext.Translations` object for retrieving
localized strings.

* Make `context` an optional parameter for all views.

* Add shortcut method to make rendering an XBlock's view with its own
runtime easier.

* Change the user field of scopes to be three valued, rather than two. `False`
becomes `UserScope.NONE`, `True` becomes `UserScope.ONE`, and `UserScope.ALL`
is new, and represents data that is computed based on input from many users.

* Rename `ModelData` to `FieldData`.

* Rename `ModelType` to `Field`.

* Split xblock.core into a number of smaller modules:

* xblock.core: Defines XBlock.

* xblock.fields: Defines ModelType and subclasses, ModelData, and metaclasses
for classes with fields.

* xblock.namespaces: Code for XBlock Namespaces only.

* xblock.exceptions: exceptions used by all parts of the XBlock project.

* Changed the interface for `Runtime` and `ModelData` so that they function
as single objects that manage large numbers of `XBlocks`. Any method that
operates on a block now takes that block as the first argument. Blocks, in
turn, are responsible for storing the key values used by their field scopes.

* Changed the interface for `model_data` objects passed to `XBlocks` from
dict-like to the being cache-like (as was already used by `KeyValueStore`).
This removes the need to support methods like iteration and length, which
makes it easier to write new `ModelDatas`. Also added an actual `ModelData`
base class to serve as the expected interface.