release 0.5.7

[chfw]

address pyexcel/pyexcel-io#66

[chfw]

in order to work with openpyxl 2.6.0, the work will be left over to v0.6.0

[codecov-io]

Codecov Report

Merging #28 into master will decrease coverage by <.01%.
The diff coverage is 98.66%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   97.76%   97.76%   -0.01%     
==========================================
  Files          12       12              
  Lines         627      626       -1     
==========================================
- Hits          613      612       -1     
  Misses         14       14

Impacted Files	Coverage Δ
pyexcel_xlsx/xlsxw.py	100% <100%> (ø)	⬆️
tests/test_formatters.py	100% <100%> (ø)	⬆️
tests/base.py	98.21% <100%> (ø)	⬆️
tests/test_hidden.py	100% <100%> (ø)	⬆️
tests/test_multiple_sheets.py	97.22% <100%> (ø)	⬆️
tests/test_filter.py	100% <100%> (ø)	⬆️
tests/test_merged_cells.py	100% <100%> (ø)	⬆️
tests/test_writer.py	97.29% <100%> (ø)	⬆️
pyexcel_xlsx/__init__.py	100% <100%> (ø)	⬆️
tests/test_bug_fixes.py	100% <100%> (ø)	⬆️
... and 2 more

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