py-bitarray: updated to 2.7.4

2.7.4:
* register `bitarray` as `abc.MutableSequence`
* cibuildwheel: update macOS version to 11 from unsupported 10.15, see
  https://github.com/actions/runner-images/issues/5583
* improve documentation regarding type when indexing single
  bitarray items

(adam)