Bump xlsxwriter from 1.2.8 to 1.3.7 (#108)

Bumps [xlsxwriter](https://github.com/jmcnamara/XlsxWriter) from 1.2.8 to 1.3.7.
- [Release notes](https://github.com/jmcnamara/XlsxWriter/releases)
- [Changelog](https://github.com/jmcnamara/XlsxWriter/blob/master/Changes)
- [Commits](jmcnamara/XlsxWriter@RELEASE_1.2.8...RELEASE_1.3.7)

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