[py] better error messages for snapshot mismatch

[nedtwigg]

• There's some unimplemented code here

  • selfie/python/selfie-lib/selfie_lib/SnapshotSystem.py

    Lines 127 to 128 in 51fefff

    	def msg_snapshot_mismatch(self, expected: str, actual: str) -> str: # noqa: ARG002
    	return self.msg("Snapshot mismatch, TODO: string comparison")

• it should be implemented by porting these
  • https://github.com/diffplug/selfie/blob/main/jvm/selfie-lib/src/commonMain/kotlin/com/diffplug/selfie/guts/SnapshotNotEqualErrorMsg.kt
  • https://github.com/diffplug/selfie/blob/main/jvm/selfie-lib/src/commonTest/kotlin/com/diffplug/selfie/guts/SnapshotNotEqualErrorMsgTest.kt

Great first issue, would be a big UX improvement.