Proposal: don't simplify unions in expand_type()

[ilevkivskyi]

Fixes #6730

Currently expand_type() is inherently recursive, going through expand_type -> make_simplified_union -> is_proper_subtype -> map_instance_to_supertype -> expand_type. TBH I never liked this, so I propose that we don't do this. One one hand, this is a significant change in semantics, but on the other hand:

• This fixes a crash (actually a whole class of crashes) that can happen even without recursive aliases
• This removes an ugly import and simplifies an import cycle in mypy code
• This makes mypy 2% faster (measured on self-check)

To make transition smoother, I propose to make trivial simplifications, like removing <nothing> (and None without strict optional), removing everything else if there is an object type, and remove strict duplicates. Notably, with these few things all existing tests pass (and even without it, only half a dozen tests fail on reveal_type()).

[JukkaL]

I like this! I think that we've been simplifying unions too aggressively. Not simplifying unions "should not" affect semantics, except for the details of revealed types (and bugs related to union types).

Accepting on the assumption that mypy_primer doesn't show meaningful regressions.

[ilevkivskyi]

The one in paasta looks like a bug. Probably we hash TypedDicts incorrectly.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

operator (https://github.com/canonical/operator)
- ops/testing.py:906: error: Argument 2 to "get" of "dict" has incompatible type "None"; expected "Mapping[str, str]"  [arg-type]
+ ops/testing.py:906: error: Argument 2 to "get" of "dict" has incompatible type "None"; expected "Union[Dict[str, str], Mapping[str, str]]"  [arg-type]

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/utilities/asyncutils.py:205: error: Argument 1 to "is_async_fn" has incompatible type "T"; expected "Callable[[VarArg(<nothing>), KwArg(<nothing>)], Awaitable[<nothing>]]"  [arg-type]
+ src/prefect/utilities/asyncutils.py:205: error: Argument 1 to "is_async_fn" has incompatible type "T"; expected "Union[Callable[[VarArg(<nothing>), KwArg(<nothing>)], <nothing>], Callable[[VarArg(<nothing>), KwArg(<nothing>)], Awaitable[<nothing>]]]"  [arg-type]

discord.py (https://github.com/Rapptz/discord.py)
- discord/automod.py:284: error: Argument "data" to "from_data" of "AutoModTrigger" has incompatible type "object"; expected "Optional[Empty]"  [arg-type]
+ discord/automod.py:284: error: Argument "data" to "from_data" of "AutoModTrigger" has incompatible type "object"; expected "Union[_AutoModerationTriggerMetadataKeyword, _AutoModerationTriggerMetadataKeywordPreset, _AutoModerationTriggerMetadataMentionLimit, Empty, None]"  [arg-type]

[ilevkivskyi]

OK, now mypy_primer looks OK. I will merge this later today if there are no objections.