Can you please add information about your OS and how you are running the tests? It passes in Travis for 3.7 branch for my last PR. Tests were added as part of https://bugs.python.org/issue33974 .

Ref : https://travis-ci.org/python/cpython/jobs/406532961#L2397

I tried the latest 3.7 branch on my Ubuntu box and tests run fine

➜  cpython git:(25326de) ./python
Python 3.7.0+ (remotes/upstream/3.7:25326de, Jul 21 2018, 09:05:57)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
➜  cpython git:(25326de) time ./python -m unittest -v test.test_tcl.TclTest.test_join
patchlevel = 8.6.5
test_join (test.test_tcl.TclTest) ... ok

----------------------------------------------------------------------
Ran 1 test in 0.013s

OK
./python -m unittest -v test.test_tcl.TclTest.test_join  0.24s user 0.01s system 97% cpu 0.249 total


Thanks