PERF: Use xlsxwriter by default (if available)
Merge pull request #5446 from jtratner/default-imports-for-excel-writers

PERF/ENH: Use xlsxwriter by default (and then fall back on openpyxl).
add optional default value to GroupBy.get_group
add test cases to test_groupby for GroupBy.get_group with a default value
make try except block python3 compatible
change test_groupby as per @jreback's suggestion
fix last commit
replace strange [False] * len(wp) with more natural wp.month == -1