Issue installation from source with pip 21.2.1

[t20100]

CI is broken with latest version of pip when installing from from source dist:

Run pip install --pre --find-links dist/ silx
Looking in links: dist/
Processing ./dist/silx-0.16.0a0.tar.gz
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
ERROR: Some build dependencies for silx from file:///home/runner/work/silx/silx/dist/silx-0.16.0a0.tar.gz conflict with the backend dependencies: Cython==3.0.0a9 is incompatible with Cython>=0.21.1.

It is also possible to reproduce locally with: pip install --pre --no-binary silx silx
Removing --pre makes it work.....

[payno]

Strange. A prerelease enforce to have Cython 3.0.0a9 ?

[t20100]

So, the issue appears with pip v21.2.1 and v21.2 but not with previous ones.
The issue disappear when not using the --pre flag or when using --pre along with --no-buils-isolation.

Finally it is also solved by removing Cython>=0.21.1 from either pyproject.toml or setup_requires in setup.py.... And I guess that is the way to go.

See pypa/pip#10222 for details.