Similarly to #3797 this allows more flexible dict-like arguments in some functions. I specifically ran into this when using da.assign_coords(other.coords) which should intuitively be allowed. But mypy complained because assign_coords required a dict. (There was no problem at runtime.)

Note that there are two changes in behaviour:

• This PR makes the code more flexible and allows iterators over tuples as well as mappings. This should cover the types that one would typically pass to dict().
• Passing the same argument via the dict like arg and kwargs now raises TypeError instead of ValueError. This matches the existing behaviour of bin, hist, etc. and more closely matches the typical error you get from bad function calls.