Typing stubs

Not urgent but just leaving this here for a future feature request. Right now if you do anything with `mypy`, you need to flag imports of `juliacall` with

```python
from juliacall import Main as jl # type: ignore
```

Because juliacall does not have type stubs available. It would be nice to have type hinting in Python for Julia objects, especially as Python moves more in this direction.

Start by reviewing the juliacall Python import shown in the issue and how mypy currently handles it. The work is complete when Python users can type-check Julia objects without adding a type: ignore comment; no files or tests are identified in the issue.

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