Support for merge and update (| and |=) in IntervalDict, following Python 3.9's changes for dict

[AlexandreDecan]

Python 3.9 will be released on October 5th. Among the chances for this version, dict now supports | and |=, respectively for merge and update. See https://www.python.org/dev/peps/pep-0584/

Since IntervalDict aims to follow the interface of a traditional dict, we should implement them as well.
|= will be easy to do, since we can simply delegate to IntervalDict.update.
For |, it is a bit more tricky, and the current plan would be to first copy the current instance, then update this copy and return it.

Since that's a quite easy issue to address, let's label it as "good first issue" ;)