change how annotation formats are listed

[NickleDave]

rewriting tests and realizing it's not easy to test formats.show

the one unit test as written now could pass even if formats.show does not return any installed formats.
i.e. it would "pass" but actually be failing silently, because formats.show would return nothing and then we would assert that nothing is in whatever formats.show returned.

there's two separate issues here:
one is how formats.show works -- it's weird that it prints instead of returning a string or something we can work with programatically / dynamicall
that's what this issue is about -- should it return a list instead

we currently test by just capturing stdout and checking what's in the captured output, which can fail silently as just described

the other is, how do we test the output is correct?
going to open another issue for that

[NickleDave]

another option would be to parse the pyproject.toml file in the tests to check whether specified formats are installed

this doesn't address testing user-installed formats but at least lets us have a single source of truth to test the built-in formats.

Feels slightly gross to rely on a file meant for development to test that something works "in prod" -- would rather find some other way to specify built-in formats

[NickleDave]

working on 4.0 and realized that if I'm adding a formats sub-package as in #109 then I can customize the module's attributes so that they are only valid annotation format classes, by leveraging changes in #99 and #105

The basic idea is what's described here:
https://realpython.com/python37-new-features/#customization-of-module-attributes

but instead of plug-ins, I can use the abc-based interface as defined in #105 / #120 in combination with a customized __dir__ and __getattr__ to only list / return classes that are registered as sub-classes of the interface