Writing "inf" silently creates invalid workbook
*Created originally on Bitbucket by [snorfalorpagus (Joshua Arnott)](https://bitbucket.org/%7B60afeab2-3636-4f2e-bd58-aefc874ab62f%7D/)*
The following code will run without exception, but creates an invalid workbook. This is caused because Excel does not appear to have a representation of infinity. The same problem occurs when trying to write a `numpy.inf` value. I'm not sure what the correct behavior is here: either raise an exception, or write "inf" as a string.
```
#!python
import openpyxl
wb = openpyxl.Workbook()
sheet = wb.create_sheet("Test")
sheet["A1"] = float("inf")
wb.save("test.xlsx")
```
Tested on openpyxl 2.4.8 and 2.5.0b1, both installed via anaconda/conda-forge.
issue