Explicit flushing of the cursor had been implemented in the "legacy"
`url_open` helper, but it was missing when going through `requests`
directly (via the `opener`), or when performing XML-RPC calls.
Fix that:
* extend `requests.Session` and `xmlrpc.client.Transport` so they take
a cursor
* move the setup of the XML-RPC clients to `setUp` so they can *get*
the cursor
* move the rest of `HttpCase.__init__` to `setUpClass` and drop the
override entirely
* remove the now-redundant flush in `url_open` (as it's done by the
`opener`)
closesodoo/odoo#77359
X-original-commit: c7bdae34690f31960d1fa95b5c3b4052540f5d07
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>