CI: 3.8 build
Travis config is a mystery to me, so this is trial-and-error until I get the hang of it.

Closes https://github.com/pandas-dev/pandas/issues/26626
see #27266 as well here For a moment I was amazed that the last "restore everything disabled during debugging" commit came back green.

Then I saw that I had mangled the Travis config so badly that Travis didn't run at all. FYI, on the NumPy mailing list they announced that they built 3.8 wheels. Looks like https://github.com/MacPython/numpy-wheels/blob/master/.travis.yml 

Maybe https://github.com/MacPython/numpy-wheels/commit/9d8a0adf9ef8ba0c42c2221b5584a9460ffe1e50 + an update of multibuild in the pandas-wheels repo would do the trick. > Maybe MacPython/numpy-wheels@9d8a0ad + an update of multibuild in the pandas-wheels repo would do the trick.

Does this replace the approach being taken in this PR? Or supplement? @TomAugspurger this approach is now working.  I'll take a look at the MacPython/multibuild approach to see if its significantly cleaner, but we have this as a fallback. Hmm, in the output I see

```
Looking in links: https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com/
Requirement already satisfied: numpy in /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages (1.18.0.dev0+ea66b1d)
```

but it isn't clear to me how we got NumPy previously. Reverted to use git clone, with comment about possible pip alternative @jreback thoughts on https://github.com/pandas-dev/pandas/pull/28730#discussion_r334575150?

3.8 is out now, so I'd like to get this in and 0.25.2 released as soon as possible (ideally by sometime tomorrow). https://github.com/pandas-dev/pandas/issues/29001 for followup.

Thanks @jbrockmendel.