Please make it possible to skip tests requiring Internet access

[mgorny]

When run in Internet-less environment, a number of tests fail currently. We would find it very helpful if tests requiring Internet access were marked somehow, so that we could cleanly skip them.

[mgorny]

These would be, as of 0.8.1:

tests/integration/test_hooks.py
tests/integration/test_placeholders.py
tests/integration/test_record_modes.py except for TestRecordNone
tests/integration/test_unicode.py
tests/regression/test_gzip_compression.py
tests/regression/test_requests_2_11_body_matcher.py

Of course, it would be better to run them against a locally-spawned server, e.g. using pytest-httpbin.