TST: Change network decorator to auto-check for network errors
TST: Remove explicit connectivity checks in test cases.

Instead, network decorator in pandas.util.testing checks
for that instead. You have to opt into failing on tests
by setting `pandas.util.testing._FORCE_NETWORK_ERROR` to `True`.
CLN: Move imports and test skip to top of file
TST: Enforce more strict rules about ignoring IOErrors with network tests