Datacite

- Updates dependency versions and adds python 3.9 support
- Changes internal definition name for affiliation in 4.3 schema

- Standardizes function names in DataCiteRESTClient. Old functions will be
depreciated in a future release

- Fixes DataCiteRESTClient attributes' type. Prefix, username and password
are always cast to string.

- Adds full support for DataCite Metadata Schema v4.2 and v4.3 XML generation.
- Uses Official DataCite JSON Schema, which has the following notable changes
from the previous schema:

- Uses "identifiers" which is a combination of the XML "identifier" and
"alternativeIdentifiers" elements
- "creatorName" is now "name"
- "contributorName" is now "name"
- "affiliations" is now "affiliation" (is still an array)
- "affilition" is now "name"
- There is no longer a funder identifier object (the identifier and type are just
elements)
- Removes Python 2 support
- Removes the old way of testing with DataCite: test mode for the MDS APIs and
the test DOI 10.5072

- Fixes schema location url for DataCite v4.1

- Adds full support for DataCite Metadata Schema v4.1 XML generation.