improve datetime perf

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benchmark with simple logger.info("hello {}", "b") with os.devnull sink, from 44s to 20s

ref: #1201

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looks like we are still running tests on 3.5, so we can't add inline typing or use f-string ...

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also， why aware_now use time module instead of using datetime_.now.astimezone and combine it as datetime？

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I think we can also include a fast path for default datetime format