build(version): bump to 0.20.0

Making the YAML serializer emit `null` again instead of blank scalars
may be considered a fix.
(It was an oversight I let it change.)
However, this is too significant to only bump the patch version.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "poetry.core.masonry.api"
 
 [tool.poetry]
 name = "remarshal"
-version = "0.19.1"
+version = "0.20.0"
 description = "Convert between CBOR, JSON, MessagePack, TOML, and YAML"
 authors = ["D. Bohdan <[email protected]>"]
 license = "MIT"