Fix remaining uses of deprecated cmp argument in attrs decorators

[matrixbot]

This issue has been migrated from #9641.

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Description

There are a handful of places left that use the deprecated cmp argument on the attr.s decorator:

• https://github.com/matrix-org/synapse/blob/0a00b7ff14890987f09112a2ae696c61001e6cf1/synapse/events/builder.py#L38
• https://github.com/matrix-org/synapse/blob/d29b71aa50f568c3b951b95f2ea3009292b95630/tests/server.py#L472
• https://github.com/matrix-org/synapse/blob/a7a379006651ea219c2618c0a47e8f4053a9e769/synapse/types.py#L421
• https://github.com/matrix-org/synapse/blob/31b1905e13f84d839f46990d7e63bafe0917dc75/synapse/crypto/keyring.py#L65

These should be fixed.

[spaetz]

Hey, is this that really decrepated? https://www.attrs.org/en/stable/api-attr.html#attr.s Says deprecated in 19.2.0 but then Changed in version 21.1.0: cmp undeprecated