Accept constant memoryviews in HashTable.lookup
xref https://github.com/pandas-dev/pandas/pull/10070
xref https://github.com/pandas-dev/pandas/issues/12813

 building an ExtensionArray for nullable ints using a `pyarrow.Array`, the underlying memoryview is marked as constant. The HashTable Cython code uses the memoryview in the `lookup` function but in a read-only fashion. Without the `const` specifier Cython raises an error that the memoryview is read-only.

Not sure how to test this in a small scope (i.e. without using `pyarrow`) or if this small change is worth mentioning in the changelog.

- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
