Log Message:
py-flit_core py_flit: updated to 3.9.0

Version 3.9

New options flit build --use-vcs and 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. Setting SOURCE_DATE_EPOCH is still \ 
recommended for properly reproducible builds.