This updates djblets.conditions.values to support typing.

BaseConditionValueField and ConditionValueFormField are now

generics, accepting a type that the fields manage. These types are

accepted in serialize_value() and returned in deserialize_value().

Other value subclasses (for model values) also take a type.
If a type is not provided, these default to Any. This is available as

part of PEP 696 and a compatible typing_extensions.
Some Python 2.7-safe code has been removed, cleaning up some tests and

implementation code.
This is the first step in making condition support type-safe, and

modernizing some of the APIs for better type compliance.

Unit tests pass.