Fix declaration of _lock, take 2

[ianthomas23]

My fix in PR #130 was wrong, although it fixed the xeus-runner problem it broke use of pyjs in emscripten-forge recipe tests. Here is the correct fix.

I've tested it locally as follows:

1. Rebuild pyjs on emscripten-forge with this patch.
2. Rebuild xeus-python on emscripten-forge using the new pyjs.
3. Rebuild xeus-pywrap on emscripten-forge using the new pyjs.
4. Now rebuilding any python package on emscripten-forge, such as contourpy, the tests pass whereas before the patch they failed with an error about the _lock.
5. Build a local jupyterlite deployment using these packages, python code such as matplotlib interactive plots run as expected without error.
6. Python and shakespearelang kernels run correctly in xeus-runner still.