[libcxx] Support Python 3.8 in the test suite

Summary: `platform.linux_distribution()` has been deprecated in Python 3.5 and removed in Python 3.8.

Reviewers: bcain, bcraig, jroelofs, EricWF, mclow.lists, ldionne

Reviewed By: jroelofs

Subscribers: dexonsmith, christof, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D72501