tests: Fix print_exception test and re-enable it on Travis CI.

Issue was with uPy: on local machine with micropython-lib installed, io
module is available.  Not the case on Travis CI, where only _io module
is available in uPy.

tests/misc/print_exception.py

@@ -1,4 +1,4 @@
-import io
+import _io as io # uPy does not have io module builtin
 import sys
 if hasattr(sys, 'print_exception'):
     print_exception = sys.print_exception

tests/run-tests

@@ -60,7 +60,6 @@ def run_tests(pyb, tests, args):
     # Some tests shouldn't be run under Travis CI
     if os.getenv('TRAVIS') == 'true':
         skip_tests.add('basics/memoryerror.py')
-        skip_tests.add('misc/print_exception.py')
 
     # Some tests shouldn't be run on pyboard
     if pyb is not None: