Merge pull request cwacek#224 from davimenezesoli/feature/223

Removed the module jsonschema.compat and use the six package instead

Merge pull request cwacek#201 from reece/200-explicit-nulls

cwacek#200: always marshal properties, even if None/null

Merge pull request cwacek#216 from cwacek/feature/213

Fix cwacek#213: Properly support total ordering for literals

Merge pull request cwacek#212 from cwacek/update-204

Apply cwacek#204

Merge pull request cwacek#197 from adriangay/importing-abcs-deprecation

fixing Python 3.8 ABC deprecation

Merge pull request cwacek#191 from cwacek/feature/fix-188

Fix 188: Check for ArrayWrapper, then collections.Sequence

Merge pull request cwacek#179 from cwacek/feature/fix-151

Feature: Fixes cwacek#151

Merge pull request cwacek#168 from cwacek/feature/fix-126

bugfix: Add a `__hash__` implementation to ProtocolBase

Merge pull request cwacek#167 from cwacek/feature/fix-165

bugfix: Properly handle attribute deletion. Fixes cwacek#165

Merge pull request cwacek#163 from cwacek/feature/fix-156

bugfix: Fix cwacek#156.