refactoring into submodules

[choldgraf]

This refactors jupyter_sphinx into a few submodules:

• ast.py
• execute.py
• thebelab.py
• utils.py

and puts the sphinx setup() function in __init__.py.

the docs build and the tests pass for me. I still want to clean up the imports, and apply a black pass, but wanted to see if this looks reasonable to others first. If folks are +1 on this, then I'll add another commit that cleans up the import statements, and applies "black" and ensures a flake8 test passes.

The one user-facing change is that the extension will now be loaded with jupyter_sphinx instead of jupyter_sphinx.execute. Perhaps there's a way we could import one into the other to temporarily keep both behaviors?

[akhmerov]

Wouldn't an execute module that's a complete copy of __init__ (perhaps with a deprecation warning) not solve it?

Otherwise backwards compatibility breaking would also be OK, but then I'd like to go sure that also the directives and the AST are stabilized before making a release.

[choldgraf]

ah good call - I just added a commit that is a little bit hacky, but I think accomplishes the same thing. Also added a deprecation warning. WDYT?

I'm not sure how to test both the old and new way...

[choldgraf]

I just cleaned up the imports a bit (so they are more well-ordered) and ran black on the codebase. I also added a simple test for the old extension activation string jupyter_sphinx.execute to make sure it still works. @akhmerov what do you think?

[akhmerov]

Thanks a lot!

[choldgraf]

woo - OK I'll try and upstream some more stuff this week