A slimmer python

### Comment:

I'm wondering if there is enough enthusiasm for having a slim python build. My main motivation is to help ensure that downstream packages that depend on python, also declare the dependencies they have on other libraries. The ones that come to mind are:

- [ ] readline
- [ ] sqlite
- [ ] tk

I'm not sure about the implementation, but maybe we can have a behavior similar to pip, where during the build process they aren't pulled in automatically, but for regular users they are? I know that the pip feature is engrained pretty deep into conda, so maybe it isn't that desirable to use it, but the behavior:
- At build time, maintainers might have to specify `python-readline`, `python-sqlite`, `python-tk` as part of their dependencies.
- At environment making time, maybe those are all pulled automatically?

With non-python packages, this has typically resulted in the creation of a "core" package. This would mean that the dependency graph might look like
````
/----- python-readline ----\------------------> readline
/------ python-sqlite -------\-----------------> sqlite
python ------------------------------------ python-core
\-------python-tk------------/-----------------> tk
```
with `tk`, `sqlite` and `readline` being removed as dependencies from `python-core`.

PS. I know this conversation is quite cyclic in nature but I figured it is time to revive it.