[Python] Support pickling DictionaryScalar

[asfimport]

Since the pa.array factory function doesn't support the creation of dictionary array pickling has not been implemented for dictionary scalars yet.

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

Related issues:

• [Python] Add custom pickle reducers for DictionaryScalar (is duplicated by)

_{Note: This issue was originally created as ARROW-9998. Please see the migration documentation for further details.}