Accept '==' operator in cubes dependencies
'==' is the operator used in the python dependencies specifications.
See PEP 440 http://legacy.python.org/dev/peps/pep-0440/#version-specifiers
Using the same operator in cubes dependencies allow to make setuptools handle
the cubes dependencies while keeping cubicweb happy about the dependencies.
Closes #4375144
[session] don't silently ignore commits
commits while in the precommit or postcommit phase are silently ignored.
This is OK in precommit since there'll be a commit afterwards anyway;
OTOH in postcommit it may surprise the developer who didn't read the
documentation for postcommit operations carefully enough. Related to
#4383922.