Took Nick Coghlan's advice about importing warnings globally in logging, to avoid...

Took Nick Coghlan's advice about importing warnings globally in logging, to avoid the possibility of race conditions: "This could deadlock if a thread spawned as a side effect of importing a module happens to trigger a warning. warnings is pulled into sys.modules as part of the interpreter startup - having a global 'import warnings' shouldn't have any real effect on logging's import time."

git-svn-id: http://svn.python.org/projects/python/trunk@67537 6015fed2-1504-0410-9fe1-9d1591cc4771