Issue #7449 part 3, test_doctest: import trace module in test_coverage()

Import trace module fail if the threading module is missing. test_coverage() is
only used if test_doctest.py is used with the -c option. This commit allows to
execute the test suite without thread support.

Move "import trace" in test_coverage() and use
test_support.import_module('trace').
1 file changed