Columns special case: FIXED pack with not enough info

[penguinolog]

In the case of weighted items without a size source, min_width can be used as width source.
Note: This behavior is only a special case.

This behavior should never be exposed in sizing to prevent incorrect render of widgets

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 7724332926

• 0 of 8 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 71.118%

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Files with Coverage Reduction	New Missed Lines	%
urwid/widget/columns.py	1	91.14%

Totals
Change from base Build 7715077568:	0.02%
Covered Lines:	8770
Relevant Lines:	12439

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💛 - Coveralls