The warnings were introduced with issue13125 and this was fixed with issue30117 . I verified it as below : 

➜  cpython git:(master) git checkout 14e976e00e65bf343ba0fca016c3c9132a843daf~1
➜  cpython git:(ce0f33d045) git clean -xdf && ./configure --with-pydebug && make -s -j4
➜  cpython git:(ce0f33d045) time ./python.exe -I -S -m test.regrtest -vv test_import test_lib2to3 >/dev/null
Warning -- files was modified by test_import
  Before: []
  After:  ['@test_41673_tmp.pyc']
/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/lib2to3/tests/test_parser.py:430: UserWarning: ParseError on file /Users/karthikeyansingaravelan/stuff/python/cpython/Lib/lib2to3/main.py (bad input: type=22, value='=', context=('', (130, 38)))
  warnings.warn('ParseError on file %s (%s)' % (filepath, err))
/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/lib2to3/tests/test_parser.py:430: UserWarning: ParseError on file /Users/karthikeyansingaravelan/stuff/python/cpython/Lib/lib2to3/tests/pytree_idempotency.py (bad input: type=22, value='=', context=('', (49, 33)))
  warnings.warn('ParseError on file %s (%s)' % (filepath, err))
./python.exe -I -S -m test.regrtest -vv test_import test_lib2to3 > /dev/null  44.31s user 1.18s system 94% cpu 47.971 total

# Fixed commit

➜  cpython git:(14e976e00e) time ./python.exe -I -S -m test.regrtest -vv test_import test_lib2to3 >/dev/null
Warning -- files was modified by test_import
  Before: []
  After:  ['@test_41839_tmp.pyc']
./python.exe -I -S -m test.regrtest -vv test_import test_lib2to3 > /dev/null  50.50s user 1.56s system 91% cpu 56.778 total

The test_import warning is also not reproducible with master as of now I will find the necessary commit and reply back in issue28655. As part of triaging I am closing this as duplicate with issue30117 as superseder which has the fix. Feel free to reopen this if needed.

Thanks