YAML config files over 4096 chars

[percevalw]

Currently, we don't support yaml config files that go over 4096 characters.

For instance, this fails:

def test_very_long_yaml_config():
    config = Config.from_yaml_str(
        """\
        a: {}
        """.format("x" * 4100)
    )
    assert config == {"a": "x" * 4100}

This is related to how we handle value parsing in our code here and how yaml loads files in chunks here