Add py.typed for PEP561 compliance

[mgrinshpon]

According to https://typing.python.org/en/latest/spec/distributing.html#packaging-type-information and PEP-561, the only thing that's missing is a little py.typed file to tell type checkers that this library is indeed properly typed (which it is!).

This will fix errors from users trying to use py-pglite with pyright and basedpyright on strict mode when annotating fixtures with py-pglite's type signatures.

Thanks for making this library. It's awesome!