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Bug fix for log format colorization (fixes `9`_).
Rationale: I'm not validating the format, I just want to extract the referenced
field names, so looking for ``%(..)`` without a trailing type specifier (and
optional modifiers) is fine here.
.. _Release 3.1.2: https://github.com/xolox/python-coloredlogs/compare/3.1.1...3.1.2
.. _9: https://github.com/xolox/python-coloredlogs/issues/9