Iters

Changes

- Changed `reduce` and `reduce_await` on `Iter[T]` and `AsyncIter[T]` to return `Option[T]`
instead of erroring on empty iterators.

Internal

- Changed `variable is marker` and `variable is no_default`
to `is_marker(variable)` and `is_no_default(variable)` respectively.

No significant changes.

Internal

- Migrated to Python 3.8.

Fixes

- Fixed `final` import to be compatible with Python 3.7.

Internal

- Migrated to using `typing-aliases` library.

Changes

- This release contains lots of breaking changes. Please refer to the API documentation.