[pkg] fix cubicweb.spec syntax (grafted from 3309ddb970594cd54529f5c205d1ea5b5da4131e)

[pkg] Use Twisted < 16.0.0 We have some test failures with 16.0.0, such as: - test_put_content (unittest_server.ETwistHTTPTC) - test_base_url (unittest_httptest.TwistedCWAnonTC) (grafted from 10c12f31d0fe38c2e8814176fc42bf2ea210efdf)

[pkg] fix cubicweb.spec syntax

[cwconfig] Use imp.load_source instead of exec() to load site_cubicweb.py from apphome Lets us return a module object and go back to the previous code in CubicWebConfiguration._load_site_cubicweb.

[cwconfig] Stop using lgc.modutils to load *config.py and *ctl.py files

[cwconfig] Stop using lgc.modutils to load site_cubicweb.py files config.load_site_cubicweb function now takes a list of cubes (instead of paths) and default to all known cubes.

[test] Use a Popen in i18n tests Instead of check_call, so that we clearly see the failure. A bit of flake8 along the way.

[forms] Fix HTableFormRenderer _render_fields method behavior when display_label is False. Indeed, now the extra <tr><th>...</th></tr> are not inserted anymore unless display_label is True. Closes #3811325.

[connection] remove a dead counter Related to #1381328.

[session] drop .is_repo_in_memory This is deader than a dead dog's bone buried down a blind alley off a dead-end street in a ghost town. Related to #1381328.