Autodoc fails on enums containing special characters

Describe the bug

Consider the following enum:

Colorspace = Enum("Colorspace", "RGB RGBA L LA 1 CMYK CMYK;I P PA")

Autodoc'ing this enum will yield the following error:

WARNING: invalid signature for autoattribute ('Colorspace.CMYK;I')
WARNING: don't know which module to import for autodocumenting 'Colorspace.CMYK;I' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)

It looks like autodoc may be unable to handle enum attributes containing the ; character.
(In Python, the attribute can be accessed by indexing: Colorspace['CMYK;I'])

How to Reproduce

(see the description)

Expected behavior

It should be possible to document enum attributes containing a semicolon character (;). Respectively any character that is not compatible with dotted access but valid in other notations (e.g. getitem indexing).

Your project

private, sorry

Screenshots

No response

OS

Linux Ubuntu 20.04

Python version

3.8.10

Sphinx version

4.5.0

Sphinx extensions

sphinx.ext.autodoc

Extra tools

No response

Additional context

No response

Start in the sphinx.ext.autodoc entry point and reproduce the issue with the provided Colorspace enum containing “CMYK;I”. Trace how autodoc parses and resolves enum attributes, then verify that names valid through getitem access are documented without invalid-signature or import warnings.