* Fix tests on python 3.6:
    * Make `cattrs` optional again
    * Don't run tests for serializers with missing optional dependencies
    * Show any skipped tests in pytest output
 * Fix redirect serialization for backends that serialize all values (DynamoDB and Redis)
    * Otherwise, the redirect value (which is just another key) will get converted into a `CachedResponse`
* Make `pickle` serializer use `cattrs` if installed
* Make `bson` serializer compatible with both `pymongo` version and standalone `bson` library
* Split up `CattrStage` and preconf converters into separate modules
* Turn preconf converters into `Stage` objects
* Add `DeprecationWarning` for previous method of using `itsdangerous`, now that there's a better way to initialize it via `SerializerPipeline`
* Remove `suppress_warnings` kwarg that's now unused
* Make `SerializerPipeline`, `Stage`, and `CattrStage` importable from top-level package (`from requests_cache import ...`)
* Add some more details to docs and docstrings

* Pass `**kwargs` to backend storage classes, split out any that are valid for the backend-specific connection function/class, and pass them to the connection
* Add intersphinx links to docs for dependencies
* Update and format some more backend class docstrings
* Remove 'Unrecognized keyword arguments' warning from `BaseStorage`
* Turn `warnings.warn` about using secret keys into a `logging.warning` (due to complaints about too many messages)

* Also allow passing a backend class (in addition to backend name and instance)
* Detect duplicate params: Backends have different keyword args equivalent to the `cache_name` positional arg, for example `db_name`. If one of these is passed as a keyword arg, it results in a `TypeError` due to duplicate args (See #94).
* For backends without dependencies installed, use a placeholder class to delay and re-raise the original `ImportError`
* Add type annotations