CI: allow failure in deploy_to_testpypi job

Today we had a messy problem with testpypi: when trying to publish to
it, the server returned a 503 Service Unavailable error. On the second
try it game the same error, then on the third try it turns out that the
second try did actually succeed, so the third try gave an error because
the files were already uploaded. Hopefully that was just an intermittent
error and not something to expect frequently in the future.

If this job finishes with failure and the fvm-formal version was alredy
deployed to testpypi, the next jobs should succeed without problems.

If this job finishes with failure and the fvm-formal was not deployed to
testpypi, we will see an error on the following stages, which should be
solvable by retrying the deploy_to_testpypi job when testpypi becomes
stable again.

Version 1.0.0rc1