bpo-40066: [Enum] update str() and format() output (GH-30582)

Undo rejected PEP-663 changes:

- restore `repr()` to its 3.10 status
- restore `str()` to its 3.10 status

New changes:

- `IntEnum` and `IntFlag` now leave `__str__` as the original `int.__str__` so that str() and format() return the same result
- zero-valued flags without a name have a slightly changed repr(), e.g. `repr(Color(0)) == '<Color: 0>'`
- update `dir()` for mixed-in types to return all the methods and attributes of the mixed-in type
- added `_numeric_repr_` to `Flag` to control display of unnamed values
- enums without doc strings have a more comprehensive doc string added
- `ReprEnum` added -- inheriting from this makes it so only `__repr__` is replaced, not `__str__` nor `__format__`; `IntEnum`, `IntFlag`, and `StrEnum` all inherit from `ReprEnum`

Lib/re.py

@@ -155,6 +155,8 @@ class RegexFlag:
     # sre extensions (experimental, don't rely on these)
     TEMPLATE = T = sre_compile.SRE_FLAG_TEMPLATE # disable backtracking
     DEBUG = sre_compile.SRE_FLAG_DEBUG # dump pattern after compilation
+    __str__ = object.__str__
+    _numeric_repr_ = hex
 
 # sre exception
 error = sre_compile.error