Write stubs with utf-8 encoding

[jorgen]

This is to ensure that you don't get encoding errors if docstrings contains odd characters like emojis.

[JelleZijlstra]

Would you mind adding a test that includes a non-ASCII character in a docstring?

[jorgen]

I had a short stab at adding a unit test. Simplest test of this as I can tell is to add it to the stubgen.test, but I'm having a hard time running this. Simplest example of showing the problem is

with open("test", "w") as f:
    f.write("\U0001F60A")

Adding encoding="utf-8" to this fixes the problem. This is on Windows. On Linux I would not imagin this being a problem.

The workaround for this is setting PYTHONUTF8=1 but I spent some time finding this, and I imagined the patch it would save someone else this time.

[jorgen]

Thank you!