Issue28082, for 3.6.0b1, modified the re module to use enum.IntFlag. As described in Issue19325, on interpreter startup site.py imports sysconfig which, on OS X, imports _osx_support which imports, among other things, re. Now that re imports enum, a number of collections modules are also imported at startup on OS X which can have a negative performance impact. test_startup_imports of test_site was designed to catch situations like this ... and does.
For 3.6.0b1, I'm going to skip the failing part of the test on OS X only. For 3.6.0b2, I'll plan to revive languishing Issue19325 to at least try to remove the usage of re and then re-enable the skipped test.
The changes for bpo-29585 to speed up startup removed the import of _osx_support, and of re, from site.py so the disabled test does not fail in 3.7 and is now re-enabled. It's not appropriate to try to backport the startup speedup changes to 3.6, so the disabled portion of the test remains disabled on 3.6.x macOS.