Thanks Serhiy. Thanks Victor for the fix. I can confirm this on 3.6. Interestingly `--fail-env-changed` flag didn't catch this but running in verbose mode failed.

`--fail-env-changed` passes

➜  cpython git:(3707bcf02b) ./python.exe -I -S -m test.regrtest -j0 --fail-env-changed test_asyncio test_ctypes test_email test_idle test_import test_importlib test_json test_lib2to3
Run tests in parallel using 6 child processes
0:00:04 load avg: 3.51 [1/8] test_import passed
0:00:07 load avg: 3.51 [2/8] test_ctypes passed
0:00:09 load avg: 3.87 [3/8] test_idle passed
0:00:19 load avg: 6.23 [4/8] test_importlib passed
0:00:27 load avg: 6.05 [5/8] test_json passed
0:00:29 load avg: 5.72 [6/8] test_email passed
running: test_asyncio (59 sec 815 ms), test_lib2to3 (51 sec 972 ms)
0:01:06 load avg: 4.33 [7/8] test_asyncio passed (1 min 4 sec) -- running: test_lib2to3 (58 sec 276 ms)
0:01:09 load avg: 4.33 [8/8] test_lib2to3 passed (1 min 244 ms)

== Tests result: SUCCESS ==

All 8 tests OK.

# warnings are present

➜  cpython git:(3707bcf02b) ./python.exe -I -S -m test.regrtest -vv test_import test_lib2to3 >/dev/null
Warning -- files was modified by test_import
  Before: []
  After:  ['@test_10093_tmp.pyc']

Total duration: 1 min 9 sec
Tests result: SUCCESS