py3: use stringutil.pprint() to print NoneType
Before this patch, when running test-debugcommands.t, we get a TypeError because
NoneType can't be converted into bytes. This patch uses stringutil.pprint() to
print the ui._colormode.
We are now close to getting test-debugcommands.t passing on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D3831
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