(backport of 6b9f547e92d8)

This fixes a subsequent refleak in test_smtplib.

Remove a sleep to fix transient test failures. Use skewfactor of -3 to
make it work on systems that have 1 second precision for time.time().

Closes #11999
Refs #13254

packaging.manifest.Manifest under Windows.  The "recursive-include"
directive now recognizes both legal path separators.