[server] Make cubicweb/server/utils.py flake8-clean

[server] Drop utils's LoopTask and TasksManager classes not used anymore Related to 17057223.

[server] replace repository's tasks manager by a scheduler Repository() does not accept anymore a 'tasks_manager' argument but rather a 'scheduler' argument which is expected to be an instance of sched.scheduler class. The drop the _tasks_manager attribute of the repository and adjust all internal usages of it. In particular, in the 'repo_stats' service we do not export 'looping_tasks' statistics anymore as there's no way to retrieve this anymore from a web instance. Closes #17057223.

[server] introduce a scheduler class to run repository "looping tasks" We just use the sched module from the standard library and introduce a tiny Python2/3 compatibility layer (more for convenience actually). The "looping" aspect of tasks (previously in LoopTask class) is re-implemeted as a `schedule_periodic_task` function. This is a reasonably thin layer as compared to LoopTask/TasksManager classes. Only the "restart" aspect of LoopTask is no longer present as I'm not sure it's worth keeping. The advantage of using this (in addition to eventually dropping our custom code) is that this scheduler class provides a `run` method that blocks the process while running tasks in its queue. So we can rely on this to have a 'scheduler' ctl command (see forthcoming patch) that would only run "looping tasks" without having to implement the "blocking" aspect ourself. Related to #17057223.

web: add options to ignore css compilation and uicache

[skeleton] call includeme from new cube

[etwist] Do not call repository's start_looping_tasks anymore and warn about this We are about to drop this method from Repository class and replace it by a blocking alternative. This is not compatible with how things currently work in a Twisted server implementation. So do not start repository "looping tasks" in Twisted server anymore and issue a warning about this. If someone is interested in restoring the "all-in-one" behavior where the repository runs within a Twisted server, they may start by implementing repository looping tasks using a Twisted mechanism such as, e.g., http://twistedmatrix.com/documents/current/core/howto/time.html and eventually provide the repository with a compatible scheduler instance so that is can register its periodic tasks. At the moment, we lack resources to do this (and maintain the Twisted server of CubicWeb in general). Related to #17057223.

[test] Flake8-clean and use stdlib unittest in cubicweb/server/test/unittest_utils.py

[web] Don't raise ProcessFormError if there is no value specified a03376213747 introduced a slight modification in None / empty string values and with it a regression where an exception is raised on empty values, while we should simply consider no value is specified and return None.

[test] Cleanup imports in unittest_form Remove unused and reorder, + use bare unittest along the way.