chore: add py.typed file

[mxschmitt]

Added a py.typed file to tell mypy (a Python linter) that we are a typed package.

Before we didn't treat

playwright-pytest/pytest_playwright/pytest_playwright.py

Line 271 in 695c3ea

class CreateContextCallback(Protocol):

as part of our public API - but turns our our customers wants to have a typed new_context function which is reasonable.

Is the name CreateContextCallback appropriate? We are still under 0.X releases - so we could rename in theory.

Fixes #244