Issue #20753: Skip PasswordProtectedSiteTestCase when Python is built without threads.
Issue #20753: Merge with 3.4.
Fixes #10541: regrtest -T is broken

 * makes test_trace tests restore the tracefunc after they run
 * write_results() in trace module will not terminate if lnotab
   cannot be found.
Issue #21778:  Clarify use of flags if PyBuffer_FillInfo() is used inside a
getbufferproc().
Merge 3.4.
Issue #21679: Prevent extraneous fstat() calls during open().  Patch by Bohuslav Kabrda.
Issue #14235: Use importlib.reload() in test_cmd.test_coverage.
Issue #14235: Merge from 3.4.