erogers-dstl

Full Error message: ```` ============================= test session starts ============================= platform win32 -- Python 3.9.4, pytest-6.2.5, py-1.11.0, pluggy-1.0.0 rootdir: C:\Users\edward\code\Stone-Soup2 collected 1175 items / 1 error / 2 skipped / 1172...

I did consider this, but thought (wrongly) it wouldn't help as requests isn't in `extras_require`, but of course, it is a requirement of Sphinx, so implicitly included. As it is...

Come to think of it: it is a requirement for `stonesoup.reader.opensky` to import, not just to run the tests, so maybe `requests` should be a (non-dev) dependency (as `ordered-set` is,...

Also: it is possible this shouldn't be in `base.py`. Would a new file be more appropriate?