typing: declare the `_phasesets` member of `phasecache` to be `Optional`
Something in this area got flagged while making the repository class visible to
pytype (instead of being typed as `Any`). A None assignment to something not
optional is wrong, and when I tried setting it to `{}` to keep it non-Optional,
some tests failed. There are checks for the attr being None elsewhere, so this
seems to have just been an oversight.