Enable pip install --editable . on macOS

[cclauss]

Could you confirm that pip install --editable . works on macOS as well now?

• Add Github Actions workflow to run Python doctests #96 (comment)

With the proposed change:
% uv venv
% source .venv/bin/activate
% uv pip install --editable .
% python
Python 3.14.6 (main, Jun 11 2026, 03:55:33) [Clang 22.1.3 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> import lua
... lg = lua.globals()
... print(f"{lg = }")
... print(f"{lg.string = }")
... print(f"{lg.string.lower = }")
... print(f"{lg.string.lower('Hello world!') = }")
... assert lg.string.lower('Hello world!') != 'Hello world!'
... assert lg.string.lower('Hello world!') == 'hello world!'
...

lg = <Lua table at 0x105eea5a0>
lg.string = <Lua table at 0xb390b8930>
lg.string.lower = <Lua function at 0x1059812a0>
lg.string.lower('Hello world!') = 'hello world!'