gh-147991: Speed up tomllib import time

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Defer regular expressions import until the first datetime, localtime or non-trivial number (other that just decimal digits) is met.

• Issue: Improve tomllib startup time #147991

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It might be interesting to replace from types import MappingProxyType with built-in frozendict. But currently, the GitHub Action CI runs mypy with Python 3.12 which doesn't have frozendict.

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I marked added constants and functions as private by adding _ prefix. I'm not sure if it's needed, all other _parser APIs are "public" (no underscore prefix).

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I disabled the optimization when mypy runs, because I failed to fix mypy otherwise.