re-use a single request session

This should unlock performance benefits by utilising HTTP keep-alive to
reuse one TCP connection between many requests. Quoth the docs:

[The Session object] across all requests [...] will use urllib3’s
connection pooling. So if you’re making several requests to the same
host, the underlying TCP connection will be reused, which can result in
a significant performance increase (see HTTP persistent connection).[^1]

[^1]: https://requests.readthedocs.io/en/latest/user/advanced/