IgwnOmniAuth raises TypeError on Debian

Any use of the IgwnOmniAuth request handler on Debian, or any platform where requests-kerberos is used in place of requests-gssapi, raises a TypeError:

$ python3 -m igwn_monitor.plugins.check_url -a any -H example.com
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/plugins/check_url.py", line 178, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/plugins/check_url.py", line 144, in main
    status, message, detail = check_url(
                              ^^^^^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/plugins/check_url.py", line 45, in check_url
    resp = get_with_auth(
           ^^^^^^^^^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/http.py", line 486, in get_with_auth
    return get(url, **request_kw)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/http.py", line 447, in get_with_any
    request_kw["auth"] = IgwnOmniAuth(
                         ^^^^^^^^^^^^^
  File "/home/duncan/git/igwn-monitoring-plugins/igwn_monitor/http.py", line 214, in __init__
    HTTPSPNEGOAuth.__init__(self, **spnego_kw)
TypeError: HTTPKerberosAuth.__init__() got an unexpected keyword argument 'opportunistic_auth'

The opportunistic_auth keyword argument is not supported in requests-kerberos (it supports something similar called force_preemptive).

However, the default for that argument is what we set our default to, so we can probably just remove the offending setdefault call.