Mark as PEP 561 (type checking) compatible

[huonw]

Is your feature request related to a problem? Please describe.

MyPy and other static type checkers are helpful for validating code. Currently syrupy doesn't work perfectly in projects that use mypy, because it is not considered PEP 561 compatible: https://mypy.readthedocs.io/en/latest/installed_packages.html#making-pep-561-compatible-packages

This means that something like:

# syrupy-typing.py
from syrupy.assertion import SnapshotAssertion

def test_foo(snapshot: SnapshotAssertion) -> None:
    assert "foo" == snapshot(match="x")

fails with

syrupy-typing.py:2: error: Skipping analyzing 'syrupy': found module but no type hints or library stubs
    from syrupy import SnapshotAssertion
    ^
syrupy-typing.py:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports

The fix for users for this is ignoring the imports (as described in the link there), such as from syrupy.assertion import SnapshotAssertion # type: ignore. However, this is unfortunate because it stops mypy and potentially other tooling from picking up the error: match instead of matcher.

Describe the solution you'd like

By adding a py.typed file as described in https://mypy.readthedocs.io/en/latest/installed_packages.html#making-pep-561-compatible-packages , the existing type annotations become available to mypy users.

The output of the above then becomes:

syrupy-typing.py:5: error: Unexpected keyword argument "match" for "__call__" of "SnapshotAssertion"; did you mean "matcher"?
        assert "foo" == snapshot(match="x")
                        ^

Describe alternatives you've considered

A separate stub file/project seems unnecessary, given there's already type annotations inline in the code.

Additional context

N/A