Json-syntax

Technically a bugfix, but it also technically breaks pickling compatibility.

* Fixes handling of private fields in attrs classes.

Release notes:

- Adds experimental support for classes derived from `typing.Generic`.
- Expands supported pythons to include 3.4
- Factors some error support into `json_syntax.errors`
- Factors some typing support into `json_syntax.types`

Maintenance notes:

- Added configuration and documentation for testing via `dephell docker` commands.
- Some cleanup of the unit tests for annotations

This adds the new string verbs to the rules in `extras.flags`, as well as making them compatible with Python 3.5.

Release notes:

* Add separate verbs for encoding and decoding strings to be used by homogenous mapping types.
* Drop `read_all` as we shouldn't try to save `init=False` keys in attrs.
* Make sure `helpers._Context` is stringifying context items.

Release notes:
- Improves extras.dynamodb
- Fixes TypedDict

New:
- New dynamodb ruleset in `extras`
- Add preliminary `TypedDict` support for when 3.8 lands.
- Adds a `RLockCache` that uses reentrant locks.
- Refactors product type (attrs, dataclass, named tuples) especially for better error messages

Potentially breaking changes:
- The `examples` directory is now `extras`
- Add `access` method to `SimpleCache` that RuleSet will call.
- Some actions in `action_v1` are expecting the new `Attribute` object instead of a tuple