gh-68164: Set the "regular file" bit in zipfile's writestr

[thatch]

• Issue: ZipFile.writestr implies non-regular files #68164

[python-cla-bot]

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[thatch]

@vstinner trivial change for writestr from 10-year-old bugreport

I left the ZipInfo default alone -- anyone using ZipInfo.from_file already sets this bit, and changing the default breaks anyone trying to set the symlink bit with | (as even cpython's test suite does)