travis: Ensure Python 3.8 is used, disable failing PyPy 2.7 build

[EricCousineau-TRI]

Resolves#2159

Lemme know if I should take out the disabling of PyPy.

Thanks to @carlsonmark for the help!

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This change is 

[EricCousineau-TRI]

Nuts... seems like Sphinx upgrade to 3.0.0, which breaks things?

[wjakob]

Looks good, but let's definitely not drop tests if there is only a single one covering that flavor of interpreter. Can you amend this PR with the fix for Python 3.8?

[wjakob]

Hi @EricCousineau-TRI -- sorry for taking a while to respond to this PR.
First, I'd like to double-check: can this be closed now, or is it still applicable? (given that some improvements to CI have recently landed, like a38e533)

[EricCousineau-TRI]

Yup! I can close it!

But just to check, I looked at a few PRs, and they only had an AppVeyor status (no TravisCI status):
#2260
#2265

Is that expected? (and would you like that to be a separate issue?)

[wjakob]

I noticed that too. Something (not me, did not touch) recently messed up the integration. Earlier today, I removed all the permissions and recreated them. I'm hoping that bumping those PRs will fix cause all of the CI to rebuilt. Note that Travis itself is unaffected, you can log in there and see all of the builds.

[EricCousineau-TRI]

Awesome, sound great - closing now!