[ty] implement auto() for StrEnum

[thejchap]

Summary

see discussion here: astral-sh/ty#876 (comment)

https://docs.python.org/3/library/enum.html#enum.StrEnum

Note Using auto with StrEnum results in the lower-cased member name as the value.

Test Plan

• new mdtest
• also, added a test to assert the (already correct) behavior for IntEnum

[github-actions]

Diagnostic diff on typing conformance tests

No changes detected when running ty on typing conformance tests ✅

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
+ src/scikit_build_core/build/_wheelfile.py:51:22: error[no-matching-overload] No overload of function `field` matches arguments
- Found 44 diagnostics
+ Found 45 diagnostics

No memory usage changes detected ✅

[sharkdp]

This is great — thank you very much!