Update pre-commit hook versions

[keewis]

pre-commit autoupdate also wants to update mypy to 0.790, but running that prints a error:

xarray/core/utils.py:466: error: Value of type variable "_LT" of "sorted" cannot be "K"

I'm not sure how to fix that, so I'll leave the upgrade to someone else.

[mathause]

See: python/mypy#9582

Maybe K should be replaced with Hashable or K = TypeVar("K", bound=Hashable) (or maybe it has to be "Comparable" as suggested in the issue).

edit: see also https://github.com/python/typeshed/blob/f35e79fd24d896678abd1b682c2b2429a546500b/stdlib/2and3/builtins.pyi#L88

[mathause]

I am not entirely sure what I say below is correct. So would be good if someone with more experience with python typing could have a look.

---

I think the solution is to add # type: ignore to

xarray/xarray/core/utils.py

Line 466 in 7ce0110

return iter(sorted(self.mapping))

---

The alternative would be to define:

class HashableSortable(Protocol):
    def __lt__(self, __other: Any) -> bool: ...
    def __hash__(self) -> int: ...

SortableHashableT = TypeVar("SortableHashableT", bound=HashableSortable)

but then we would probably need to replace all instances of "Dict[Hashable, int]" with "Dict[SortableHashable, int]".

Unfortunately we cannot do

class Sortable(Protocol):
    def __lt__(self, __other: Any) -> bool: ...

SortableT = TypeVar("SortableT", bound=HashableSortable, covariant=True)

because covariant type variable cannot be used as a parameter.

[keewis]

let's continue this in #4571