Editable installs broken with pip 22.1

# Problem

All CI jobs will probably fail right now if using `pip` 22.1 with the tell-tale error of:

```
pip._vendor.packaging.markers.UndefinedEnvironmentName: 'extra' does not exist in evaluation environment.
```

## References
- observed on #724
- probably due to https://github.com/pypa/pip/issues/11110
- it's suggested the "clever" `_version.py` and `version_info` stuff triggers `flit` to do more install than it's ready to do...

## Approaches

- wait for it to resolve itself
- `pip` or `flit` might fix something
- accept dupliaction
- in `pyproject.toml`
- remove `version` from `dynamic`
- add explicit version there
- add a test, etc. to verify the versions are in sync
- ???

Start by reproducing the editable-install failure with pip 22.1 and inspect pyproject.toml alongside the _version.py and version_info handling mentioned in the issue. Check the linked pip issue and flit behavior before choosing between dependency behavior and duplicated version metadata. Done means editable installs no longer produce the UndefinedEnvironmentName error and a test verifies the relevant version values stay in sync.

Written by the indexing model from the issue text.