PERF: make io imports lazy in PY37+
This shaves about 3.5-4% off of our import time.

If we want to continue down this path, we can do the same thing for `eval` and `reshape.api` pretty easily.  Probably more than that, but I haven't looked that far.

FWIW: a full run of `asv` (on one commit, not `continuous`) imports pandas just over 5000 times, so these couple-dozen milliseconds add up.