Fix length calculation when adding extended ascii filenames - #32
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python/cpython#150091 changed the default non-utf8 encoding from 'ascii' to 'cp437' for encoding zip filenames. This means that characters that could be encoded in cp437 but not ascii would be encoded in the zip data using cp437 vs. previously they woud've used utf-8. Note that non-ascii characters that are valid cp437 characters will always take up a single byte when encoded as cp437, but multiple when encoded using utf-8. Because `len(zipstream)` assumes that filenames are encoded using only ascii or utf-8, this resulted in the calculated length being off by (at least) 1 byte for every cp437, non-ascii character in all the filenames. The fix in this commit is to override `ZipInfo._encodeFilenameFlags` to revert Python's change and force filenames to only use ascii/utf-8. The change was made to support modifying existing zips with cp437-encoded filenames which isn't relevant to this project. And while technically cp437 is the standard encoding for zipfile names, using the more strict ascii subset is less likely to be misinterpreted by unarchivers since they won't have to guess at what extended ascii encoding was used - it'll always be strict ascii or utf-8.
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Fixes #31