Replace use of simplejson with built-in json module

[sphuber]

aiida-core ships with the aiida.common.json module which provides the dump, dumps, load and loads functions that have the same interface as the json module of the standard library. These custom wrappers are used internally by AiiDA to serialize and deserialize JSON before it is written to files in the repository or in the database. For the implementation it uses simplejson which is the externally maintained version of json to provide backwards compatibility of old Python versions. Since we no longer support those Python versions, it would be good to investigate if we can remove these custom wrappers and libraries and simply rely on the standard library.

[soma2000-lang]

@sphuber can i work on this