juliacall segfaults when loading a package that uses PythonCall

juliacall always segfaults on me when I try to load a package that uses PythonCall.

This should reproduce the error:

```python
import juliapkg

juliapkg.add("BATPythonCall", "dc6699a0-bee0-4ed4-84d6-ad34e0e2513f", rev = "loaddebug", url = "https://github.com/bat/BATPythonCall.jl.git")

juliapkg.resolve()
juliapkg.status()

from juliacall import Main as jl

jl.seval("using BATPythonCall")
```

results in

```
free(): invalid pointer

signal (6): Aborted
in expression starting at none:1
gsignal at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
abort at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
```

BATPythonCall.jl (on branch "loaddebug") is basically an empty package that only does `using PythonCall`. Removing `using PythonCall` makes the segfault disappear.

CC @philippeller

Start by running the Python reproduction with juliacall, juliapkg, and BATPythonCall on its loaddebug branch, then compare it with the case where using PythonCall is removed. Trace the package-loading entry point involved in `jl.seval("using BATPythonCall")`; done means the package loads without the invalid-pointer abort.

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