Fix skipping tests requiring the requests module

Fix skipping tests requiring the requests module

The intended purpose of the @pytest.mark.requests decorator was to cause
Python-based parts of the "statschannel" system test to be skipped if
the requests Python module is not available.  However, both
tests-json.py and tests-xml.py contain a global "import requests"
statement which triggers ImportError exceptions during test
initialization if the requests module is not available.  In other words,
the @pytest.mark.requests decorator serves no useful purpose.

Since all tests in both tests-json.py and tests-xml.py depend on the
requests Python module, employ pytest.importorskip() to ensure the
Python-based parts of the "statschannel" system test are skipped when
the requests module is not available.  Remove all occurrences of the
@pytest.mark.requests decorator (and all associated code) to prevent
confusion.