test: add test to ensure that stub files are valid

[tlambert03]

hey @pthom

this test might help to avoid regressions similar to #349

here's what it would have shown prior to #349

E           AssertionError: Found 1 .pyi files with syntax errors:
E
E           SyntaxError parsing implot/internal.pyi:
E           invalid syntax. Perhaps you forgot a comma? (internal.pyi, line 2241)

[pthom]

Hi @tlambert03

This is brilliant! It not only raises an error but show where it is located.
I just merged your PR.

Thanks a lot!