CLN: Remove unnecessary ExcelWriterMeta metaclass

Only `__new__` is really necessary. And subclasses just have to deal
with being passed an 'engine' keyword argument + a tad bit of style
cleanup.
Merge pull request #5403 from jtratner/remove-excel-metaclass

CLN: Remove unnecessary ExcelWriterMeta metaclass
ENH: read_excel: try converting numeric to int

All Excel numeric data is stored as floats, so have to convert
specifically. Changing default because it's suprising to save something
with what looks like a row/column of integers and get a column of floats
instead. (especially because it can lead to annoying Float64Indexes)