Doc: expose more prominently `const` class attributes
They are closer to the actual behavior.
State explicitly that function-based class attributes are also computed only once at initialization
fix(pyo3-macros): allow pyclass named Probe
The `*` import was shadowing the name of the pyclass.
The PyMethods trait was only defined for the multiple-pymethods feature,
but it cannot be conditionally imported inside the macro.
I just defined the trait unconditionally.
Fixes #4792.