- update to 3.9.0:
* New options :option:`flit build --use-vcs` and :option:`flit
build --no-use-vcs` to enable & disable including all
committed files in the sdist. For now --use-vcs is the
default, but this is likely to change in a
future version, to bring flit build in line with standard
build frontends like python -m build
* Sdist file names, and the name of the top-level folder in an
sdist, are now normalised, in accordance with PEP 625
* A statically defined version number can now be parsed from
files called version.py, _version.py or __version__.py inside
a packge, as well as from __init__.py, so executing code is
required in fewer cases
* Fix setting the flag for regular files in zip metadata
* The timestamp embedded in the gzip wrapper for sdists now
defaults to a fixed date, so building an sdist twice on the
same machine should produce identical results, even without
any special steps.

- skip install tests for better interoperability with PEP 668