chore(deps): upgrade dependency msgspec to ~=0.21.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
msgspec	~=0.20.0 → ~=0.21.0

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Release Notes

jcrist/msgspec (msgspec)

v0.21.0: Version 0.21.0

Compare Source

• Fix a segfault on Python 3.13+ that could occur when creating structs that contained a __dict__, instead of only using __slots__ (#​960).
• Improve type annotations for msgspec.UNSET (#​975).
• Fix hashing of decoded tuples on Python 3.14 (#​980).
• Add ref_template arg to msgspec.json.schema (#​983).
• Add missing libm linkage to some architectures (#​993).
• Fix memory leak when decoding sets (#​991).
• Add uniqueItems annotation to set/frozenset JSON schemas (#​984).
• BREAKING: Call __post_init__ from msgspec.structs.replace/copy.replace (#​1000).

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