Merge stable into master

`six.moves.collections_abc` was added in six 1.13.0: https://github.com/benjaminp/six/blob/master/CHANGES

#246

Adds a new base class, `uplink.retry.RetryBackoff`, which can be extended to implement custom backoff strategies. Subclasses can override three methods:

- `get_timeout_after_response(request, response)`: Returns the number of seconds to wait before retrying the request, or ``None`` to indicate that the given response should be returned.
- `get_timeout_after_exception(request, exc_type, exc_val, exc_tb)`: Returns the number of seconds to wait before retrying the request, or ``None`` to indicate that the given exception should be raised.
- `handle_after_final_retry()`: Handles any clean-up necessary following the final retry attempt.

An instance of a `RetryBackoff` subclass can be provided through the `backoff` argument of the `@retry` decorator.

#238

…229)

Fix ``@returns.json`` to cast JSON response (or field referenced by the ``key`` argument) using the ``type`` argument when the given type is callable. This restores behavior that was inadvertently changed in v0.9.3.

Example:

```python
  @uplink.returns.from_json(key=("data", 0, "size"), type=int)
  @uplink.get("/users/{user}/repos")
  def get_first_repo_size(self, user):
      pass
```

```python
assert github.get_first_repo_size("prkumar") == 300
```

#215