[Draft] Add Oauth support

[mdavis-xyz]

This PR adds a few things.

• adds OAuth support, since the basic auth support is being deprecated. Add OAuth identification #67
• Improves detection and documentation of invalid timespans, as per Is there a two day constraint? #76
• Raises Python exception in the case of an HTTP error. Previous behavior was to just return None. This is a breaking change I suppose (though not covered by unit tests, I think). But returning None instead of an error makes this library extremely difficult to use. see Calling example returns None #72
• Document how to display logs to the Python logger
• Use a requests.Session(), so that the TLS and TCP connection is re-used across multiple requests, for performance reasons, and to simplify authentication handling
• rename the url_post argument to an internal function, because it's for a GET request, so the name was misleading
• the tests can now be run with just python test*.py (At first I thought the tests were passing, then I realised that they weren't actually being run.)

Note that all the get_state tests fail for me, even on the master branch. I think I don't have sufficiently privileged auth credentials.

And now I'm getting 503 errors for most calls, so I can't be sure this is correct. That's why I've marked this PR as Draft. (I'm still uploading it so you can see, because I might not have time to get back to this for another week.)

[jkua]

User report here - thanks for this! I've been using the Python client for the last few days.

It mostly works, but I occasionally get:

File "/Users/jkua/projects/XXXXX/.venv/lib/python3.13/site-packages/opensky_api.py", line 316, in __init__
   raise ValueError("Authentication Failure (OAuth token generation)") from ex

NameError: name 'ex' is not defined. Did you mean: 'hex'?

Certainly, ex doesn't exist, but it also appears that I'm occasionally getting token generation issues.

[mdavis-xyz]

I've pushed a fix now, to the same branch.

[mdavis-xyz]

Just FYI, it seems like the maintainers of this repo don't respond to many pull requests, so I don't expect this to be merged.

I've made a fork with this and several other improvements here:

https://github.com/mdavis-xyz/opensky-api/tree/fork

Install it with:

pip install git+https://github.com/mdavis-xyz/opensky-api.git@fork