Typing spec should be clearer that type checkers are not expected to support PEP 3141

Beginne mit der Seite „Special cases for float and complex“ der typing spec und vergleiche ihre Formulierung mit den verlinkten Abschnitten aus PEP 484 und PEP 3141. Stelle klar, dass die ABCs aus PEP 3141 nicht die empfohlenen numerischen Annotationen sind und dass Type-Checker sie möglicherweise nicht unterstützen; der aktualisierte Abschnitt sollte die bestehende Erklärung zur numerischen Kurzschreibweise beibehalten.

With regards to the numeric tower, PEP 484 states:

PEP 3141 defines Python’s numeric tower, and the stdlib module numbers implements the corresponding ABCs (Number, Complex, Real, Rational and Integral). There are some issues with these ABCs, but the built-in concrete numeric classes complex, float and int are ubiquitous (especially the latter two :-).

Rather than requiring that users write import numbers and then use numbers.Float etc., this PEP proposes a straightforward shortcut that is almost as effective: when an argument is annotated as having type float, an argument of type int is acceptable; similar, for an argument annotated as having type complex, arguments of type float or int are acceptable. This does not handle classes implementing the corresponding ABCs or the fractions.Fraction class, but we believe those use cases are exceedingly rare.

This is a very useful passage to link to, because:

• It clearly (albeit tersely) states that the PEP-3141 ABCs are problematic
• It clearly states the separate solution that PEP-484 proposes for dealing with numeric types
• It frames the PEP-484 solution in opposition and contrast to the PEP-3141 solution

The parallel passage in the typing spec currently states this:

Python’s numeric types complex, float and int are not subtypes of each other, but to support common use cases, the type system contains a straightforward shortcut: when an argument is annotated as having type float, an argument of type int is acceptable; similar, for an argument annotated as having type complex, arguments of type float or int are acceptable.

Since PEP-484 is a historical document rather than a piece of living documentation, it would be great if the typing spec could state as clearly as PEP-484 that the PEP-3141 ABCs are not the recommended way of annotating numeric types in Python, and that type checkers may not necessarily (and in fact probably won't ever) support them.