PythonCall.jl 0.9.19 and 0.9.20 seem broken on julia-nightly on GH actions

**Affects:** PythonCall

**Describe the bug**

When running the tests for [BraketSimulator.jl](https://github.com/amazon-braket/BraketSimulator.jl), which use PythonCall.jl, on GitHub actions using `julia-nightly`, I get the following error:

```
LoadError: UndefVarError: `pyconvert` not defined in `PythonCall`
```
Even though as far as I can tell, PythonCall.jl does export this function. It's only occuring on GH actions, I can't reproduce it locally with a checkout of Julialang `master`.

Please include the steps required to reproduce the bug. This should include not just code but **all** the steps required to reproduce the bug

**Your system**
Please provide detailed information about your system:
- Occurs on Windows, Ubuntu, and macOS runners
- GH actions `julia-nightly` and PythonCall.jl `0.9.19` and `0.9.20`.

**Additional context**
Add any other context about the problem here.

Start by reproducing the BraketSimulator.jl tests on GitHub Actions across Windows, Ubuntu, and macOS with julia-nightly and PythonCall.jl 0.9.19 or 0.9.20. Investigate the `PythonCall.pyconvert` undefined error and confirm the result with the affected test runs; done means the cause is identified and the nightly tests pass or the incompatibility is clearly documented.

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