Misc fixes

[dimbleby]

another small collection of small fixes

• simplify and correct handling of metadata fixtures in tests: metadata should be read as bytes, not text, and then the code is common
• some highly pedantic typechecking fixes after I accidentally ran pyright over the codebase. (pyright gives plenty of other warnings, mostly complaining about tomlkit, the things changed here are the ones that looked halfway interesting)
• update build. See build: add editable to dist type pypa/build#764 re the typing, their next release will once again consider "editable" to be ok
• transitive dependencies are "transitive" not "transient"
• remove some python 3.7 typechecking trickery that is no longer necessary

[github-actions]

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