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').

Lib/test/test_doctest.py

@@ -2468,8 +2468,9 @@ def test_main():
         # Check the doctest cases defined here:
         test_support.run_doctest(test_doctest, verbosity=True)
 
-import trace, sys
+import sys
 def test_coverage(coverdir):
+    trace = test_support.import_module('trace')
     tracer = trace.Trace(ignoredirs=[sys.prefix, sys.exec_prefix,],
                          trace=0, count=1)
     tracer.run('reload(doctest); test_main()')