Add type stub for pytest

The only part of the API we reference directly from the code in `tests/`
is the bare minimum `pytest.raises`, so that's all I've implemented here.

Apparently pytest are working on providing type stubs, but it's not
at a usable state yet: pytest-dev/pytest#3342

stubs/pytest.pyi

@@ -0,0 +1,15 @@
+"""
+Type stubs for pytest.
+
+Note that stubs are only written for the parts that we use.
+"""
+
+from typing import ContextManager, Type
+
+
+# This function actually has more arguments than are specified here, and the
+# context manager actually yields a _pytest._code.ExceptionInfo instead of None.
+# We don't use either of these features, so I don't think its worth including
+# them in our type stub. We can always change it later.
+def raises(exc_type: Type[Exception]) -> ContextManager[None]:
+    ...