When defining fields for a resource's response, or fields accepted

during a request, we previously made use of some standard Python 2.7

primitives, such as unicode, file, int, and some special values

like a list or tuple containing possible choices for a value. These

were only handled in Djblets by the @webapi_request_fields decorator,

which had built-in support for specific types. Resources were free to

list fields as well, but they didn't do anything special with them

(though consuming applications could, for documentation purposes).
The real problem was that these types weren't all compatible with

Python 3. For instance, file does not exist. It was also just not very

future-proof in general, and while it served us well for a while,

there's more we can do with a new model.
This change introduces a new concept of field types, which are classes

that know how to do things like validate and normalize a value coming in

from a request, and generate a description of the type (intended for

documentation purposes). With these, @webapi_request_fields no longer

needs to know about specific types, and can therefore be more simple and

more maintainable.
Field types plug right into a field info dictionary's 'type' item,

where the old legacy types were. When instantiated (which happens on

demand, not during field list definition), they're passed the field info

dictionary, which may contain other data useful for that field type

(such as a list of choices, or a resource path/class).
There's a handful of built-in types, some geared more for documentation

purposes (like ResourceFieldType) and some for more specialized

handling of common request values (like DateTimeFieldType).
Existing dictionaries with legacy types going into

@webapi_request_fields are still supported. Legacy types are

automatically converted into the new types the first time the function

is called.
Going forward, we'll be able to use this to help further separate out

field validation/normalization from API logic in Review Board and in

Djblets.

Djblets unit tests pass.
Converted all of Review Board to the new field types (upcoming change)

and verified that all unit tests pass and all docs generated correctly.
Built the new docs. Didn't see any errors.