fix type for TestCase.assertIn (#2186)
fix type for TestCase.assertIn (#2186)

* fix type for TestCase.assertIn

This does essentially `assert member in container`, so we want a `Container`, not an `Iterable`.

This came up in https://github.com/ambv/black/pull/288/files/68e9d426a86edc187a3f58ea889a2002620f0de6..0bbee43d60dfc16d8bbdd0bbdaad754a2c8c7ec0#r192525658.

* use any for assertIn

euresti/typeshed,li-dan/typeshed