[false positive] "Unsupported converter" raised for methods

Bug Report

mypy reports an error when a method is used as an attr converter.

To Reproduce

import attr
@attr.s(auto_attribs=True, slots=True)
class UpperString():
    """Uppercase string."""
    value: str = attr.ib(converter=str.upper)

Expected Behavior

str.upper is a named method. It takes a str (self, usually) and returns a `str.

Actual Behavior

"Unsupported converter, only named functions and types are currently supported"

Your Environment

• Mypy version used: 0.910 (latest)
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: latest
• Operating system and version: all

Reproduce the example with mypy 0.910 and inspect how attr converter expressions are classified, focusing on why the named method str.upper is rejected. Done means the example type-checks without the false positive and a regression test covers method converters.