Stem unit tests hang after doctest on macOS

[atagar]

Migrated from ticket 30595.

  doctest...                                           success (0.08s)
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

​https://travis-ci.org/teor2345/stem/jobs/536613170#L364