Wheel-inspect

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- Support Python 3.10
- Remove the upper bound on the `readme_renderer` dependency

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- When verifying a `RECORD`, directory entries listed therein are now checked
for existence.
- Remove dependency on `pkg_resources`
- Dropped support for Python 3.5
- Support Python 3.9
- Drop `read_version` build dependency
- Support wheels whose filenames and `.dist-info` directories use different
casings/spellings for the project name and/or version
- Better validation errors for wheels with missing or improper `.dist_info`
directories

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- Added an `inspect_dist_info_dir()` function for inspecting bare, unpacked
`*.dist-info` directories
- Added a `DIST_INFO_SCHEMA` schema describing the return value of
`inspect_dist_info_dir()`
- Renamed `SCHEMA` to `WHEEL_SCHEMA`; the value remains available under the
old name for backwards compatibility
- The `wheel2json` command now accepts directory arguments and inspects them
with `inspect_dist_info_dir()`

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- **Bugfix**: Now *actually* discard *all* empty keywords
- Split off the wheel filename processing code into its own package,
[wheel-filename](https://github.com/jwodder/wheel-filename). Wheel-Inspect
currently re-exports `ParsedWheelFilename` and `parse_wheel_filename()` from
this library in order to maintain API compatibility with earlier versions,
but this may change in the future.
- Adjusted the schema to indicate that `.dist_info.metadata.description` may be
`null`
- Binary extension modules and modules located in `*.data/{purelib,platlib}`
are now included in `.derived.modules`

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- Drop support for Python 3.4
- Update `property-manager` dependency, thereby eliminating a
DeprecationWarning

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- When splitting apart comma-separated keywords, trim whitespace and discard
any keywords that are empty or all-whitespace
- Support Python 3.8