* fix docstring in generate_package_index() refering to "public" keys as "signing" keys
* use explicit kwargs in push_to_url()
* simplify url_util.parse() per tgamblin's suggestion
* replace standardize_header_names() with the much simpler get_header()
* add some basic tests
* update s3_fetch tests
* update S3 list code to strip leading slashes from prefix
* correct minor warning regression introduced in #11117
* add more tests
* flake8 fixes
* add capsys fixture to mirror_crud test
* add get_header() tests
* use get_header() in more places
* incorporate review comments
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