Added note to documentation about copy_reg rename.

Renamed copy_reg to copyreg in the standard library, to avoid
spurious warnings and ease later merging to py3k branch. Public
documentation remains intact.

Factored the import guard in test_py3kwarn.TestStdlibRemovals into
a context manager, namely test_support.CleanImport.

one.

I am amazed that the old code, for inserting null-bytes, actually
worked. Who wrote that thing? Oh, it is me... doh.

Allow multiple repositories in .pypirc; see http://wiki.python.org/moin/EnhancedPyPI
for discussion.

The patch is slightly revised from Tarek's last patch: I've simplified
the PyPIRCCommand.finalize_options() method to not look at sys.argv.
Tests still pass.

DeprecationWarning.

test.test_support.import_module().