Use factory for requests session objects
The `make_session()` factory creates session objects with default values
for timeouts, number of retries, etc., which can be overridden when
calling it.
Create such (long-lived) session objects in the constructors of the
individual toddlers using requests to access remote APIs. Pull in
module-global functions into the toddler class where necessary and adapt
tests to accommodate.
Also, mock out the factory function in the `toddler´ fixture for testing.
Signed-off-by: Nils Philippsen <nils@redhat.com>