[3.12] gh-113320: Reduce the number of dangerous getattr() calls when constructing protocol classes (#113401)

[AlexWaygood]

• Only attempt to figure out whether protocol members are "method members" or not if the class is marked as a runtime protocol. This information is irrelevant for non-runtime protocols; we can safely skip the risky introspection for them.
• Only do the risky getattr() calls in one place (the runtime_checkable class decorator), rather than in three places (_ProtocolMeta.init, _ProtocolMeta.instancecheck and _ProtocolMeta.subclasscheck). This reduces the number of locations in typing.py where the risky introspection could go wrong.
• For runtime protocols, if determining whether a protocol member is callable or not fails, give a better error message. I think it's reasonable for us to reject runtime protocols that have members which raise strange exceptions when you try to access them. PEP-544 clearly states that all protocol member must be callable for issubclass() calls against the protocol to be valid -- and if a member raises when we try to access it, there's no way for us to figure out whether it's a callable member or not!

(cherry-picked from commit ed6ea3e)

Closes #113320

• Issue: Should typing be calling getattr on arbitrary classes?  #113320