Ensure S3 tests are mocked
Anaconda owns the `pandas-test` S3 bucket that we use for some of our tests. They're doing some spring cleaning, and would like to remove this bucket sometime next week.

We'll need to make sure that all of our tests are mocked.

From what I can tell, `pandas/tests/io/parser/test_network::TestS3` is not mocked.

We also have an ASV benchmark that hits S3, whose goal is to ensure that we don't read the whole file when doing `read_csv(..., nrows=10)`. I think we could remove that and replace it with a mocked fixture that asserts something about the number of bytes read.