Support Py_LIMITED_API in separate package as a dependency to PyCall.jl

The Python C ABI/API has stability guarantees for a limited number of structs and functions.

https://www.python.org/dev/peps/pep-0384/

It'd be nice to support this as a separate package and use that package as a dependency in PyCall.jl. This separate package can implement the Py_LIMITED_API structs and functions without invoking the Python interpreter.

A (very early) prototype is located here:

https://github.com/kdheepak/Python.jl/blob/33b98177b0e624588a3f46ccfa96b317d57f1201/src/Python.jl

The motivation for this is so that we can build a shared object from a Julia file using [PackageCompiler.jl](https://github.com/JuliaLang/PackageCompiler.jl) that is also a Python module. See screenshot below as an [example](https://github.com/JuliaLang/PackageCompiler.jl/blob/f40fb987c7e5f3d6c6ad9245f957af90b78bf575/hello.jl):

No linking to the Python library occurs at compile time and the Python module does not embed a Python interpreter; it is a pure Julia shared library. This allows one to create a Python module without having Python installed and the structs and functions described in the Py_LIMITED_API would be sufficient to make this work.

Taking this a step further, we could allow for something like the following in the future (See https://github.com/yglukhov/nimpy and https://github.com/PyO3/PyO3 for similar projects in Nim and Rust):

**Write Julia code**

```julia
# hello.jl
using Python

@pycallable function hello(name::String)::Cvoid
println("Hello $name!")
end
```

**Compile**

```bash
$ julia --project juliac.jl -vasij `pwd`/hello.jl > /dev/null
```

**Run in Python**

```bash
$ ipython
Python 3.7.3 (default, Mar 27 2019, 16:54:48)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.8.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import hello

In [2]: hello.hello("John")
Hello John!

In [3]:
```

My understanding is that this would pose some challenges with the current setup in PyCall.jl since PyCall.jl initializes a interpreter in [`__init__`](https://github.com/JuliaPy/PyCall.jl/blob/9f4387535936f275e3582ebdc87cc2ef1f210030/src/pyinit.jl#L128) and when PackageCompiler runs, the references to that interpreter are embedded in the shared library.

Comments?