fix: make pypika compatible with PEP 561

[fedetibaldo]

My local mypy setup had a few complaints about this package. In particular, as there are no stubs for pypika available on typeshed, it lamented the missing typings. However, by looking at the source, it's clear that the type annotations are indeed available—they're just inlined.

To fix that, I added a py.typed file inside my .venv/.../pypika folder. It worked, so I'm replicating those changes upstream. My reference for the implementation was the mypy documentation. I've never published a package, so this is a stab in the dark for me. Hopefully, I hit the target 😜

Cheers

[AzisK]

Hi @fedetibaldo, this is already being solved in #666. I see that you added it in the MANIFEST.in file. I am not sure if we need it here, let's read and ask around