Fix some things to make inventory script perform better.

[wenottingham]

• use requests sessions, for HTTP pipelining

Given the large number of API requests this script makes, this improves things quite a bit.

• only write cache once

This is less of a raw performance issue, but there was not any obvious to me reason why it was rewriting the cache for every single host.

[wenottingham]

... if accepted, this should also go into https://github.com/ansible/ansible/pull/19510

[agx]

Merged. Thanks!