feature: Merge value when attribute is first annotated then assigned

[pawamoy]

Is your feature request related to a problem? Please describe.

In the following example:

dataclass_opts: dict[str, bool]
if sys.version_info >= (3, 10):
    dataclass_opts = {"slots": True}
else:
    dataclass_opts = {}

...the value of dataclass_opt will be None.

Describe the solution you'd like

Should we visit the if nodes instead and merge the assigned value to the previously registered annotated-only attribute?

Describe alternatives you've considered

/

Additional context

In could help value inference in #240.

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[pawamoy]

If we implement this, we should first add an option to hide attribute values in mkdocstrings-python, since we've been recommending to delay assignments as a solution:

a: str  # no value shown by mkdocstrings-python
a = "hello"