[testlib] introduce a RepoAccess class to easily create connection and request Each RepoAccess hold a session for a user and three helper function to help create Connection, ClientConnection of WebRequest related to this session. related to #2920299

[repoapi] make ClientConnection.__enter__ return self This allow the standard idiom:: with repoapi.connect(repo, login='babar', passwork='elephant') as cnx: cnx.execute(…)

[documentation] describe repoapi and web side change. Short version explaining what object replace what and that BC existes for a few version.

[testlib] use internal_cnx instead of internal_session internal_session is deprecated.

[repository] add an ``internal_cnx`` method to replace ``internal_session`` Accessing the repo through a Session is deprecated. We need an easy replacement for ``internal_session``. This API change was a good occasion to stop disabling integrity hook by default. This is huge source of bug in user-code. related to #2503918

[connection] enforce that a connection must be open to be used The same than for ClientConnection, we ensure the connection is used inside a its context. .. note:: We may rely on that for ClientConnection and remove de dedicated code in Client Connection but I prefer the current explicite and duplicated version for now.

use standalone Connection to Client Connection