Some serialization fixes & updates:

* 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

1 files changed, 3 insertions, 2 deletions

diff --git a/requests_cache/serializers/pipeline.py b/requests_cache/serializers/pipeline.py
index 9cd2fcf..e16799f 100644
--- a/requests_cache/serializers/pipeline.py
+++ b/requests_cache/serializers/pipeline.py
@@ -4,8 +4,7 @@ from ..models import CachedResponse
 
 
 class Stage:
-    # Generic utility class for aliasing dumps and loads to
-    # other methods
+    """Generic class to wrap serialization steps with consistent ``dumps()`` and ``loads()`` methods"""
 
     def __init__(self, obj: Any, dumps: str = "dumps", loads: str = "loads"):
         self.obj = obj
@@ -14,6 +13,8 @@ class Stage:
 
 
 class SerializerPipeline:
+    """A sequence of steps used to serialize and deserialize response objects"""
+
     def __init__(self, steps: List):
         self.steps = steps
         self.dump_steps = [step.dumps for step in steps]