Feature: add Literal for join method "how" argument, modernize typing syntax (#345)

This small PR focus on:

1) new Literal type for DuckDBPyRelation.join "how" argument, according
to the following documentation:
https://duckdb.org/docs/stable/clients/python/relational_api#join
This will provide autocomplete and an immediate overview of the
supported options.

2) Replacement of all deprecated 3.9 typing.Union and typing.List usages
by their modern counterparts, standardizing this across the file
(sometimes it was the old syntax, sometimes the modern one).

I wanted to keep this separated from my next PR who will focus on the
Expressions inputs typing, to keep them focused and make review easier
(since this one modify lines scattered across the file.)

Author: evertlammerts