Investigate entrypoints instead of pkgutils.itermodules()

[Carreau]

https://pypi.org/project/entrypoints/ was written by Thomas Kluyver to speedup and simplify finding and loading entrypoints.

pkgutils.itermodules() can be slow as it has to enumerate all the packages IIRC, so entrypoints might be a good alternative. It also allow to have backends as part of a package with a different name which is convenient for flexibility.

[teoliphant]

This looks like a good idea. The only issue is extending the number of dependencies that EPython has -- which we'd like to keep minimal right now.