Require SupportsBool instead of bool for comparisons.

[randolf-scholz]

Fixes #12562.

Essentially a reopen of #12939 with additional tests. This PR was closed due to a misunderstanding.

Note that an earlier comment by JelleZijlstra suggested that SupportsBool is equivalent to object in practice. This does not seem to quite be the case: Code sample in pyright playground

from typing import Protocol, runtime_checkable

@runtime_checkable
class SupportsBool(Protocol):
    def __bool__(self) -> bool: ...

x: SupportsBool = True
y: SupportsBool = object()  # mypy: ❌ pyright:  ❌

assert isinstance(x, SupportsBool)  # ✅
assert not isinstance(y, SupportsBool)  # ✅

Indeed, despite the documentation mentioning object.__bool__, this method is not defined on object. Rather, the truthiness of an object is defined roughly like:

if hasattr(obj, "__bool__"):
    return obj.__bool__()
if hasattr(obj, "__len__"):
   return bool(len(obj))
return True

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

colour (https://github.com/colour-science/colour)
- colour/plotting/colorimetry.py:565: error: Value of type variable "SupportsRichComparisonT" of "min" cannot be "floating[_16Bit] | floating[_32Bit] | float64"  [type-var]
- colour/plotting/colorimetry.py:566: error: Value of type variable "SupportsRichComparisonT" of "max" cannot be "floating[_16Bit] | floating[_32Bit] | float64"  [type-var]

jax (https://github.com/google/jax)
+ jax/_src/pallas/mosaic/pipeline.py:1779: error: Unused "type: ignore" comment  [unused-ignore]

ibis (https://github.com/ibis-project/ibis)
- ibis/expr/types/relations.py:2986: error: Value of type variable "SupportsRichComparisonT" of "sorted" cannot be "NumericValue | float"  [type-var]

arviz (https://github.com/arviz-devs/arviz)
- arviz/stats/ecdf_utils.py:72: error: Value of type variable "SupportsRichComparisonT" of "min" cannot be "Any | float64"  [type-var]

[srittau]

Thanks!